ath9k: Cleanup interface handling

* Do not set/clear TSF when adding/deleting an interface.
  This should be done when the BSS is set up and should also
  take into account the existence of other interfaces.

* Set opmode explicitly.

* ANI setup needs to be decided based on multiple interfaces.
  This can be done via the bss_info_changed() callback.

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