ath9k: Cleanup bss_info_changed callback

* Remove a code chunk dealing with operating mode changes.
  As noted, all such policy changes are to be done in
  add_interface.

* Remove pointless check for empty BSSID.
  Also, remove mode checks - mac80211 does all the needed checks.

* Handle enabling/disabling beacon transmission properly.

* Handle beacon interval changes for AP mode.
  The original code depended on config_interface() to update
  the HW TSF. Since that callback has been removed, handle
  it properly.

* Remove unneeded code dealing with key/privacy.

* Set the chainmasks to 1x1 for IBSS when the BSSID is set.
  This was happening uncondionally before.

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