Revert "ath9k: Calculate sleep duration"

This reverts commit 09ebb810.

ath9k_hw_set_sta_beacon_timers() configures AR_TIM_PERIOD with
the beacon interval. Before this commit, the sleepduration was
never greater than the beacon interval. But now, the behavior
has changed. For example, with an AP that uses a beacon interval of 100:

ath: phy9: next beacon 61128704
ath: phy9: beacon period 204800
ath: phy9: DTIM period 204800

If the sleepduration is calculated based on the listen time, then
the bmiss threshold should also be changed since the HW would
be in sleep state for a longer time, but that is not done currently.

To avoid configuring a higher beacon interval based on the sleepduration,
revert to the original behavior. Power consumption is not a
problem since PS is disabled in ath9k anyway.

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