iwl3945: use iwl3945_tx_cmd instead of iwl_tx_cmd

The patch replaces iwl_tx_cmd with iwl3945_tx_cmd to complete
transitions introduced by "iwlwifi: use iwl_cmd instead
of iwl3945_cmd"

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>