ath9k: keep btcoex period in milliseconds

btcoex periord is converted into micro seconds during initialization
and converted back to milli seconds while starting timer. As MCI code
handles btcoex period in msec, lets keep the btcoex timer in msec and
convert them into other form whenever needed.

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