AP_Motors: don't setup oneshot for a zero output mask

libraries/AP_Motors/AP_Motors_Class.cpp

@@ -107,7 +107,8 @@ void AP_Motors::rc_set_freq(uint32_t mask, uint16_t freq_hz)
     hal.rcout->set_freq(mask, freq_hz);
     if ((_pwm_type == PWM_TYPE_ONESHOT ||
          _pwm_type == PWM_TYPE_ONESHOT125) &&
-        freq_hz > 50) {
+        freq_hz > 50 &&
+        mask != 0) {
         // tell HAL to do immediate output
         hal.rcout->set_output_mode(AP_HAL::RCOutput::MODE_PWM_ONESHOT);
     }