ath9k: cancel xmit poll work at stop() callback

ath9k: cancel xmit poll work at stop() callback

We forgot to cancel this work at the stop() callback.

------------[ cut here ]------------
WARNING: at net/mac80211/util.c:511 ieee80211_queue_delayed_work+0x3a/0x40 [mac80211]()
Hardware name: 6460DWU
queueing ieee80211 work while going to suspend
Modules linked in: <-- snip -->
Pid: 5124, comm: phy0 Tainted: G        W  2.6.31-rc3-wl #4
Call Trace:
 [<ffffffffa03adcda>] ? ieee80211_queue_delayed_work+0x3a/0x40 [mac80211]
 [<ffffffff810552b8>] warn_slowpath_common+0x78/0xd0
 [<ffffffff81055394>] warn_slowpath_fmt+0x64/0x70
 [<ffffffff814ed2c9>] ? thread_return+0x3e/0x635
 [<ffffffffa03adcda>] ieee80211_queue_delayed_work+0x3a/0x40 [mac80211]
 [<ffffffffa0297690>] ath_tx_complete_poll_work+0xc0/0x100 [ath9k]
 [<ffffffffa02975d0>] ? ath_tx_complete_poll_work+0x0/0x100 [ath9k]
 [<ffffffff81069b68>] worker_thread+0x178/0x260
 [<ffffffff8106ecc0>] ? autoremove_wake_function+0x0/0x40
 [<ffffffff810699f0>] ? worker_thread+0x0/0x260
 [<ffffffff8106e89e>] kthread+0x9e/0xb0
 [<ffffffff8101302a>] child_rip+0xa/0x20
 [<ffffffff8106e800>] ? kthread+0x0/0xb0
 [<ffffffff81013020>] ? child_rip+0x0/0x20

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>