ath9k: Fix NoA start time calculation

ath9k: Fix NoA start time calculation

The start time field in the NoA attribute needs to be
updated based on the TSF timer when an absence notification
is sent by the P2P GO. When two channel contexts are active,
continuous, cyclic NoA is announced by setting the count value to 255,
but the start time is updated only once, for one beacon and
the same value is sent in all subsequent beacons, even
though the timestamp keeps moving.

Fix this by removing the check for 'periodic_noa_duration'
and assign the interface's start_time/duration values directly
when there is more than one active context.

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