iwlwifi: make broadcast station addition generic

Add function pointer for broadcast station addition so that we can call it
in from iwlcore at a later time. We only distinguish between iwlagn and
iwl3945 broadcast station addition. For the iwl3945 station addition we add
that function to iwlcore since that is where most station functionality
resides, making it part of iwl3945 will require significant code
reorganization that will dilute station management functionality. This
seems to be an efficient solution.

It may seem as though we are removing error checking when adding the 3945
broadcast station but this error checking was never really necessary since
the function returns the station id and the broadcast station id is always
set.

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