ath5k: Add watchdog for stuck TX queues

Since we do not know any better solution to the problem that TX queues can get
stuck, this adds a timer-based watchdog, which will check for stuck queues and
reset the hardware if necessary.

Ported from ath9k commit 164ace38.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>