ath9k_hw: Fix AR_RTC_KEEP_AWAKE usage

The default value that is being programmed to AR_RTC_KEEP_AWAKE
is 0x2, there is no need to program it manually in various
functions.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>