iwl3945: add debugfs to 3945

Patch adds debugfs to 3945. Also fix debugfs registration in iwlagn
to return error code if it fails.

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>