ath9k: Remove unused workaround

The workaround for ASPM/L0s is needed only for AR9485 1.0,
which was never sold and is not supported by ath9k.

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