ath9k: Do an AHB reset before doing RTC reset

Doing an RTC reset when DMA is active may corrupt memory,
make sure no DMA is active at this moment by doing an
AHB reset.

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