ath9k_htc: fix panic on packet injection using airbase-ng tool.

ath9k_htc: fix panic on packet injection using airbase-ng tool.

This should fix the oops which occurs during the packet injection
on monitor interface.

EIP is at ath9k_htc_tx_start+0x69/0x220 [ath9k_htc]
 [<f84dc8ea>] ? invoke_tx_handlers+0xa5a/0xee0 [mac80211]
 [<f82c84f4>] ? ath9k_htc_tx+0x44/0xe0 [ath9k_htc]
 [<f84db7b8>] ? __ieee80211_tx+0xf8/0x190 [mac80211]
 [<f84dce0d>] ? ieee80211_tx+0x9d/0x1a0 [mac80211]
 [<f84dcfac>] ? ieee80211_xmit+0x9c/0x1c0 [mac80211]
 [<f84dd1b5>] ? ieee80211_monitor_start_xmit+0x85/0xb0 [mac80211]
 [<c04c30cd>] ? dev_hard_start_xmit+0x1ad/0x210
 [<c04b97c2>] ? __alloc_skb+0x52/0x130
 [<c04d7cd5>] ? sch_direct_xmit+0x105/0x170
 [<c04c5e9f>] ? dev_queue_xmit+0x37f/0x4b0
 [<c0567e1e>] ? packet_snd+0x21e/0x250
 [<c05684a2>] ? packet_sendmsg+0x32/0x40
 [<c04b4c63>] ? sock_aio_write+0x113/0x130
 [<c0207934>] ? do_sync_write+0xc4/0x100
 [<c0167740>] ? autoremove_wake_function+0x0/0x50
 [<c02f4414>] ? security_file_permission+0x14/0x20
 [<c0207ad4>] ? rw_verify_area+0x64/0xe0
 [<c01e6458>] ? handle_mm_fault+0x338/0x390
 [<c0207cd5>] ? vfs_write+0x185/0x1a0
 [<c058db20>] ? do_page_fault+0x160/0x3a0
 [<c0208512>] ? sys_write+0x42/0x70
 [<c01033ec>] ? syscall_call+0x7/0xb

Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>