ath9k: Increase max listen interval

Earlier the listen interval is used to decide switching between
operating and off-channels during bgscan and to improve throughput,
the listen interval is reduced to 1. After optimiztion in scan
state machine, listen period is not used for decision making and
hence reverting it back to original value.

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