ath9k: Fix kernel panic on unplugging the device

when the device is yanked out ath_pci_remove starts doing the cleanups,
unregistering the hardware etc. so we should bail out immediately when
we get drv_flush callback from mac80211 when the card is being unplugged.
the panic occurs after we had associated to an AP.

	EIP: 0060:[<fb315b00>] EFLAGS: 00010246 CPU: 0
	EIP is at ath_reset+0xa0/0x1c0 [ath9k]
	EAX: 00000000 EBX: 000697c0 ECX: 00000002 EDX: f3c3ccf0
	ESI: 00000000 EDI: 00000000 EBP: f43e7b78 ESP: f43e7b50
 	DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
 	Process kworker/u:2 (pid: 182, ti=f43e6000 task=f3c3c7c0
	task.ti=f43e6000)
 	Stack:
 	0000002a 00000000 00000000 003e7b78 0000000f eaaa8500
	ffffffea eaaa97c0
 	eaaaa000 00000001 f43e7ba8 fb315d23 f99e7721 ecece680
	eaaac738 eaaa8500
 	eaaaa020 000000c8 000000c8 00000000 eaaa8d58 eaaa8500
	f43e7bd0 fb080b29
 	Call Trace:
	[<fb315d23>] ath9k_flush+0x103/0x170 [ath9k]
	[<fb080b29>] __ieee80211_recalc_idle+0x2c9/0x400
	[mac80211]
	[<fb080c8e>] ieee80211_recalc_idle+0x2e/0x60 [mac80211]
	[<fb07aa73>] ieee80211_mgd_deauth+0x173/0x210 [mac80211]
	[<fb084559>] ieee80211_deauth+0x19/0x20 [mac80211]
	[<f99dda53>] __cfg80211_mlme_deauth+0xf3/0x140
	[cfg80211]
	[<c0633d00>] ? __mutex_lock_common+0x1f0/0x380
	[<f99e1b5d>] __cfg80211_disconnect+0x18d/0x1f0
	[cfg80211]
	[<f99c8199>] cfg80211_netdev_notifier_call+0x159/0x5c0
	[cfg80211]
	[<c0608a64>] ? packet_notifier+0x174/0x1f0
	[<c0639202>] notifier_call_chain+0x82/0xb0
	[<c0170d8f>] raw_notifier_call_chain+0x1f/0x30
	[<c053b86c>] call_netdevice_notifiers+0x2c/0x60
	[<c0182184>] ? trace_hardirqs_on_caller+0xf4/0x180
	[<c053b8ec>] __dev_close_many+0x4c/0xd0
	[<c053ba2d>] dev_close_many+0x6d/0xc0
	[<c053bb53>] rollback_registered_many+0x93/0x1c0
	[<c018221b>] ? trace_hardirqs_on+0xb/0x10
	[<c053bc95>] unregister_netdevice_many+0x15/0x50
	[<fb07f83b>] ieee80211_remove_interfaces+0x7b/0xb0
	[mac80211]
	[<fb06a14b>] ieee80211_unregister_hw+0x4b/0x110
	[mac80211]
	[<fb311a4a>] ath9k_deinit_device+0x3a/0x60 [ath9k]
	[<fb31eed6>] ath_pci_remove+0x46/0x90 [ath9k]
	[<c03b4ac4>] pci_device_remove+0x44/0x100
	[<c043eb54>] __device_release_driver+0x64/0xb0
	[<c043ec67>] device_release_driver+0x27/0x40
	[<c043deeb>] bus_remove_device+0x7b/0xa0
	[<c043c491>] device_del+0xf1/0x180
	[<c043c530>] device_unregister+0x10/0x20
	[<c03afafe>] pci_stop_bus_device+0x6e/0x80
	[<c03afb72>] pci_remove_bus_device+0x12/0xa0
	[<c03c2f29>] pciehp_unconfigure_device+0x89/0x180
	[<c0181e54>] ? mark_held_locks+0x64/0x100
	[<c063390f>] ? __mutex_unlock_slowpath+0xaf/0x140
	[<c03c1f84>] pciehp_disable_slot+0x64/0x1b0
	[<c03c2850>] pciehp_power_thread+0xd0/0x100
	[<c0164ad0>] ? process_one_work+0x100/0x4d0
	[<c0164b4c>] process_one_work+0x17c/0x4d0
	[<c0164ad0>] ? process_one_work+0x100/0x4d0
	[<c03c2780>] ? queue_interrupt_event+0xa0/0xa0
	[<c01662bb>] worker_thread+0x13b/0x320
	[<c018221b>] ? trace_hardirqs_on+0xb/0x10
	[<c0166180>] ? manage_workers+0x1e0/0x1e0
	[<c016a654>] kthread+0x84/0x90
	[<c016a5d0>] ? __init_kthread_worker+0x60/0x60
	[<c063d106>] kernel_thread_helper+0x6/0x10

Cc: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>