ath9k: Fix Notice of Absence issues

* The index has to incremented only when advertising
  a new NoA schedule.

* Switch to non-periodic NoA when starting a scan operation
  and multiple channel contexts are active.

* Make sure that periodic NoA is advertised again when
  scan ends. Since the offchannel timer moves the offchannel
  state to IDLE after the GO operating channel becomes
  active, use a flag "force_noa_update" to update the
  NoA contents.

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