ath5k: fix CAB queue operation

We need to process tx descriptors for all queues (currently main tx
queue and cabq) which may have triggered the TX completion interrupt.
Otherwise, the queues can get stuck after sending a few frames.

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