ath9k: Fix TX queue draining

When TX DMA termination has failed, the HW has to be reset
completely. Doing a fast channel change in this case is insufficient.
Also, change the debug level of a couple of messages to FATAL.

Cc: stable@kernel.org
Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>