ath9k: Calculate sleep duration

Right now sleep duration is configured as beacon interval. It should be
the multiple of beacon interval by listen period which helps to
reduce station power consumption.

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

diff --cc drivers/net/wireless/ath/ath9k/common-beacon.c
Simple merge