igb: Unset IGB_FLAG_HAS_MSIX-flag when falling back to msi-only

Prior to cd14ef54d25 (igb: Change to use statically allocated array for
MSIx entries), having msix_entries different from NULL was an indicator
that MSIX is enabled.
In igb_set_interrupt_capabiliy we may fall back to MSI-only. Prior to
the above patch msix_entries was set to NULL by
igb_reset_interrupt_capability.

However, now we are checking the flag for IGB_FLAG_HAS_MSIX and so the
stack gets completly confused:

[   42.659791] ------------[ cut here ]------------
[   42.715032] WARNING: CPU: 7 PID: 0 at net/sched/sch_generic.c:264 dev_watchdog+0x15c/0x1fb()
[   42.848263] NETDEV WATCHDOG: eth0 (igb): transmit queue 0 timed out
[   42.923253] Modules linked in:
[   42.959875] CPU: 7 PID: 0 Comm: swapper/7 Not tainted 3.14.0-rc2-mptcp #437
[   43.043184] Hardware name: HP ProLiant DL165 G7, BIOS O37 01/26/2011
[   43.119215]  0000000000000108 ffff88023fdc3da8 ffffffff81487847 0000000000000108
[   43.208165]  ffff88023fdc3df8 ffff88023fdc3de8 ffffffff81034e7d ffff88023fdc3dd8
[   43.297120]  ffffffff813fff10 ffff880236018000 ffff880236b178c0 0000000000000008
[   43.386071] Call Trace:
[   43.415303]  <IRQ>  [<ffffffff81487847>] dump_stack+0x49/0x62
[   43.484174]  [<ffffffff81034e7d>] warn_slowpath_common+0x77/0x91
[   43.556049]  [<ffffffff813fff10>] ? dev_watchdog+0x15c/0x1fb
[   43.623759]  [<ffffffff81034f2b>] warn_slowpath_fmt+0x41/0x43
[   43.692511]  [<ffffffff813fff10>] dev_watchdog+0x15c/0x1fb
[   43.758141]  [<ffffffff813ffdb4>] ? __netdev_watchdog_up+0x64/0x64
[   43.832091]  [<ffffffff8103cd04>] call_timer_fn+0x17/0x6f
[   43.896682]  [<ffffffff8103cebe>] run_timer_softirq+0x162/0x1a2
[   43.967511]  [<ffffffff81038520>] __do_softirq+0xcd/0x1cc
[   44.032104]  [<ffffffff81038689>] irq_exit+0x3a/0x48
[   44.091492]  [<ffffffff81026d43>] smp_apic_timer_interrupt+0x43/0x50
[   44.167525]  [<ffffffff8148c24a>] apic_timer_interrupt+0x6a/0x70
[   44.239392]  <EOI>  [<ffffffff8100992c>] ? default_idle+0x6/0x8
[   44.310343]  [<ffffffff81009b31>] arch_cpu_idle+0x13/0x18
[   44.374934]  [<ffffffff81066126>] cpu_startup_entry+0xa7/0x101
[   44.444724]  [<ffffffff81025660>] start_secondary+0x1b2/0x1b7
[   44.513472] ---[ end trace a5a075fd4e7f854f ]---
[   44.568753] igb 0000:04:00.0 eth0: Reset adapter
[   46.206945] random: nonblocking pool is initialized
[   46.465670] irq 44: nobody cared (try booting with the "irqpoll" option)
[   46.545862] CPU: 7 PID: 0 Comm: swapper/7 Tainted: G        W    3.14.0-rc2-mptcp #437
[   46.640610] Hardware name: HP ProLiant DL165 G7, BIOS O37 01/26/2011
[   46.716641]  ffff8802363f8c84 ffff88023fdc3e38 ffffffff81487847 00000000a03cdb6d
[   46.805598]  ffff8802363f8c00 ffff88023fdc3e68 ffffffff81068489 0000007f81825400
[   46.894539]  ffff8802363f8c00 0000000000000000 0000000000000000 ffff88023fdc3ea8
[   46.983484] Call Trace:
[   47.012714]  <IRQ>  [<ffffffff81487847>] dump_stack+0x49/0x62
[   47.081585]  [<ffffffff81068489>] __report_bad_irq+0x35/0xc1
[   47.149295]  [<ffffffff81068683>] note_interrupt+0x16e/0x1ea
[   47.217006]  [<ffffffff8106679e>] handle_irq_event_percpu+0x116/0x12e
[   47.294075]  [<ffffffff810667e9>] handle_irq_event+0x33/0x4f
[   47.361787]  [<ffffffff81068c95>] handle_fasteoi_irq+0x83/0xd1
[   47.431577]  [<ffffffff81003d5b>] handle_irq+0x1f/0x28
[   47.493047]  [<ffffffff81003567>] do_IRQ+0x4e/0xd4
[   47.550358]  [<ffffffff8148b06a>] common_interrupt+0x6a/0x6a
[   47.618066]  <EOI>  [<ffffffff8100992c>] ? default_idle+0x6/0x8
[   47.689016]  [<ffffffff81009b31>] arch_cpu_idle+0x13/0x18
[   47.753605]  [<ffffffff81066126>] cpu_startup_entry+0xa7/0x101
[   47.823397]  [<ffffffff81025660>] start_secondary+0x1b2/0x1b7
[   47.892146] handlers:
[   47.919301] [<ffffffff812fbd7d>] igb_intr

So, this patch unsets the flag to indicate that we are not using MSIX.
This patch does exactly this: Unsetting the flag when falling back to MSI.

Fixes: cd14ef54d25b (igb: Change to use statically allocated array for MSIx entries)
Cc: Carolyn Wyborny <carolyn.wyborny@intel.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>