fix pwm_out: respect _num_outputs for calling up_pwm_servo_set()

_num_outputs is set according to the mode_pwmX call, which is required
for camera triggering.
This fixes camera triggering via PWM, as it avoids overwriting the channel.

src/drivers/pwm_out/PWMOut.cpp

@@ -558,7 +558,7 @@ bool PWMOut::updateOutputs(bool stop_motors, uint16_t outputs[MAX_ACTUATORS],
 
 	/* output to the servos */
 	if (_pwm_initialized) {
-		for (size_t i = 0; i < num_outputs; i++) {
+		for (size_t i = 0; i < math::min(_num_outputs, num_outputs); i++) {
 			up_pwm_servo_set(i, outputs[i]);
 		}
 	}