ath9k: stop btcoex on device suspend

During suspend, the device will be moved to FULLSLEEP state.
As btcoex is never been stopped, the btcoex timer is running
and tries to access hw on fullsleep state. Fix that.

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