iwlwifi: split adding broadcast station from others

In preparation for some station management changes we split the addition of
a broadcast station from the other stations. Later we will rely on mac80211
to direct all management (addition/removal) of all stations except the
broadcast station.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>