ath9k_hw: fix power state for MCI

Program MCI related power registers only if MCI interrupts
are enabled. This could help to reduce power consumptions when
WLAN alone is enabled in BT coex chips.

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