ath5k: fix locking in tx_complete_poll_work

ath5k_reset must be called with sc->lock.  Since the tx queue
watchdog runs in a workqueue and accesses sc, it's appropriate
to just take the lock over the whole function.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>