ath9k: fix fullsleep power consumption when BTCOEX is enabled

As soon as the interface brought up, btcoex timer starts running
eventhough the interface is in idle state and WLAN chip is moved
to full sleep mode. There is no point in running btcoex timer when
the wlan interface is in sleep mode and also it might consumes
more power on WLAN idle unassociated state. So lets stop the
btcoex when wlan is idle state.

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