ath6kl clear the MMC_PM_KEEP_POWER for cutpower case

ath6kl: If wow deepsleep pno suspend failure driver will choose cutpower.
in this scenario MMC_PM_KEEP_POWER should be cleared so that mmc will power
off the ar6000

kvalo: remove extra parenthesis

Signed-off-by: Ming Jiang <mjiang@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>