iwl3945: rename tx to tx_cmd

Rename iwl3945_tx_cmd variable tx to tx_cmd for better
readability.

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>