ath9k: Cleanup beacon logic

* The beaconing status routine is not required, since in
  multi-VIF cases the HW beacon parameters should not be
  re-configured.

* Remove SC_OP_TSF_RESET - when a beaconing interface comes
  up the first time, the TSF has to be reset.

* Simplify ath9k_allow_beacon_config().

* Handle setting/clearing the SWBA interrupt properly.

* Remove the TSF mangling in IBSS mode, it is not required.

* General code cleanup.

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