ath9k: Fix AHB reset for AR9280

The commit "ath9k: Do an AHB reset before doing RTC reset"
fixed RTC reset issue for AR9280 2.0 chipsets and above.
The fix is valid for all AR9280 chipsets.

Signed-off-by: Vivek Natarajan <vnatarajan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>