iwlwifi: enable packet injection for iwl3945

This patch enables packet injection on iwl3945 devices. Tested with
packetspammer and aireplay-ng.

Signed-off-by: Gábor Stefanik <netrolller.3d@gmail.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>