ath9k_hw: fix ASPM setting for AR9003

The AR_WA register should not be read when in sleep state so
add a variable we can stash its value into for when we need
to set it. Additionally the AR_WA_D3_TO_L1_DISABLE_REAL
(bit 16) needs to be removed.

Cc: Aeolus Yang <aeolus.yang@atheros.com>
Cc: Madhan Jaganathan <madhan.jaganathan@atheros.com>
signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>

Signed-off-by: John W. Linville <linville@tuxdriver.com>