[media] em28xx: fix oops at em28xx_dvb_bus_ctrl()

em28xx is oopsing with some DVB devices:

[10856.061884] general protection fault: 0000 [#1] SMP
[10856.067041] Modules linked in: rc_hauppauge em28xx_rc xc5000 drxk em28xx_dvb dvb_core em28xx videobuf2_vmalloc videobuf2_memops videobuf2_core rc_pixelview_new tuner_xc2028 tuner cx8800 cx88xx tveeprom btcx_risc videobuf_dma_sg videobuf_core rc_core v4l2_common videodev ebtable_nat ebtables nf_conntrack_ipv4 nf_defrag_ipv4 xt_CHECKSUM be2iscsi iscsi_boot_sysfs iptable_mangle bnx2i cnic uio cxgb4i cxgb4 tun bridge cxgb3i cxgb3 stp ip6t_REJECT mdio libcxgbi nf_conntrack_ipv6 llc nf_defrag_ipv6 ib_iser rdma_cm ib_addr xt_conntrack iw_cm ib_cm ib_sa nf_conntrack ib_mad ib_core bnep bluetooth iscsi_tcp libiscsi_tcp ip6table_filter libiscsi ip6_tables scsi_transport_iscsi xfs libcrc32c snd_hda_codec_realtek snd_hda_intel snd_hda_codec snd_hwdep snd_seq snd_seq_device snd_pcm tg3 snd_page_alloc snd_timer
[10856.139176]  snd ptp iTCO_wdt soundcore pps_core iTCO_vendor_support lpc_ich mfd_core coretemp nfsd hp_wmi crc32c_intel microcode serio_raw rfkill sparse_keymap nfs_acl lockd sunrpc kvm_intel kvm uinput binfmt_misc firewire_ohci nouveau mxm_wmi i2c_algo_bit drm_kms_helper firewire_core crc_itu_t ttm drm i2c_core wmi [last unloaded: dib0070]
[10856.168969] CPU 1
[10856.170799] Pid: 13606, comm: dvbv5-zap Not tainted 3.9.0-rc5+ #26 Hewlett-Packard HP Z400 Workstation/0AE4h
[10856.181187] RIP: 0010:[<ffffffffa0459e47>]  [<ffffffffa0459e47>] em28xx_write_regs_req+0x37/0x1c0 [em28xx]
[10856.191028] RSP: 0018:ffff880118401a58  EFLAGS: 00010282
[10856.196533] RAX: 00020000012d0000 RBX: ffff88010804aec8 RCX: ffff880118401b14
[10856.203852] RDX: 0000000000000048 RSI: 0000000000000000 RDI: ffff88010804aec8
[10856.211174] RBP: ffff880118401ac8 R08: 0000000000000001 R09: 0000000000000000
[10856.218496] R10: 0000000000000000 R11: 0000000000000006 R12: 0000000000000048
[10856.226026] R13: ffff880118401b14 R14: ffff88011752b258 R15: ffff88011752b258
[10856.233352] FS:  00007f26636d2740(0000) GS:ffff88011fc20000(0000) knlGS:0000000000000000
[10856.241626] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[10856.247565] CR2: 00007f2663716e20 CR3: 00000000c7eb1000 CR4: 00000000000007e0
[10856.254889] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[10856.262215] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[10856.269542] Process dvbv5-zap (pid: 13606, threadinfo ffff880118400000, task ffff8800cd625d40)
[10856.278340] Stack:
[10856.280564]  ffff88011ffe8de8 0000000000000002 0000000000000000 ffff88011ffe9b00
[10856.288191]  ffff880118401b14 00ff88011ffe9b08 ffff880100000048 ffffffff8112a52a
[10856.295893]  0000000000000001 ffff88010804aec8 0000000000000048 ffff880118401b14
[10856.303521] Call Trace:
[10856.306182]  [<ffffffff8112a52a>] ? __alloc_pages_nodemask+0x15a/0x960
[10856.312912]  [<ffffffffa045a002>] em28xx_write_regs+0x32/0xa0 [em28xx]
[10856.319638]  [<ffffffffa045a221>] em28xx_write_reg+0x21/0x30 [em28xx]
[10856.326279]  [<ffffffffa045a2cc>] em28xx_gpio_set+0x9c/0x100 [em28xx]
[10856.332919]  [<ffffffffa045a3ac>] em28xx_set_mode+0x7c/0x80 [em28xx]
[10856.339472]  [<ffffffffa03ef032>] em28xx_dvb_bus_ctrl+0x32/0x40 [em28xx_dvb]

This is caused by commit c7a45e5b4f8c2f96cd242ae1b1c06e7fb19a08d0,
that added support for two I2C buses. A partial fix was applied
at 3de09fbbfaa521e68675bd30cfece252c4856600, but it doesn't cover
all cases, as the DVB core fills fe->dvb->priv with adapter->priv.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>