ath9k: fix power consumption on network sleep when BTCOEX is enabled

The chip is waken up for every 45ms in btcoex timer cycle to
for sharing the radio between BT and WLAN. Whenever the wlan
interface is in network sleep mode, do not schedule hw timers.
This could reduce power consumption on idle associated state.

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