iwl3945/iwlwifi: unify add_station function

Patch unifies the add_station function for 3945 and iwlwifi drivers.

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