iwl3945: Select correct sta ID from find_station()

The find_station routine needs to look at the IWL_AP_ID entry if we're a STA.
Currently, it only looks for STA entries which causes HW crypto to fail.

Signed-off-by: Samuel Ortiz <samuel.ortiz@intel.com>
Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>