ath9k_hw: Fix system hang when resuming from S3/S4

The bit 6 & 7 of AR_WA (0x4004) should be enabled only
for the chips that are supporting L0s functionality
while resuming back from S3/S4.

Enabling these bits for AR9280 is causing system hang
within a few S3/S4-resume cycles.

Cc: stable@kernel.org
Cc: Jack Lee <jlee@atheros.com>
Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>