ath9k: Fix TX poll routine

Disable the TX hang monitoring routine when doing a scan.
Monitoring for a hung situation is not really necessary during
a scan run.

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