ath9k: Do a full reset for AR9280

AR9280 requires a full reset during channel change and HW reset.
Currently, a fast channel change is done. This patch fixes
this bug.

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>