HAL_PX4: fixed non-contiguous motor outputs

this fixes tricopter with chan3 never set

libraries/AP_HAL_PX4/RCOutput.cpp

@@ -435,6 +435,8 @@ void PX4RCOutput::_send_outputs(void)
             for (int i=to_send-1; i >= 0; i--) {
                 if (_period[i] == 0 || _period[i] == PWM_IGNORE_THIS_CHANNEL) {
                     to_send = i;
+                } else {
+                    break;
                 }
             }
         }