iwlwifi: unify station management

This patch unifies 3945 and AGN station management
It also removes useless struct iwl_station_mgmt ops
and cleanups a bit the interface

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Tested-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>