ath9k: avoid queueing hw check work when suspended

The following issue was reported.

WARNING: at net/mac80211/util.c:599 ieee80211_can_queue_work.isra.7+0x32/0x40 [mac80211]()
Hardware name: iMac12,1
queueing ieee80211 work while going to suspend
Pid: 0, comm: swapper/0 Tainted: PF          O 3.8.2-206.fc18.x86_64 #1
Call Trace: Mar 16 09:39:17 Parags-iMac kernel: [ 3993.642992]  <IRQ>
[<ffffffff8105e61f>] warn_slowpath_common+0x7f/0xc0
[<ffffffffa0581420>] ? ath_start_rx_poll+0x70/0x70 [ath9k]
<ffffffff8105e716>] warn_slowpath_fmt+0x46/0x50
[<ffffffffa045b542>] ieee80211_can_queue_work.isra.7+0x32/0x40

Fix this by avoiding to queue the work if our device has
already been marked as suspended or stopped.

Reported-by: Parag Warudkar <parag.lkml@gmail.com>
Tested-by: Parag Warudkar <parag.lkml@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>