iwl3945: use iwl_cmd_queue_free

iwl_cmd_queue_free needs to be used to free up the cmd_queue,
as TFD slots for cmd_queue and tx_queue are different.

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>