fix pwm_out: respect _num_outputs for calling up_pwm_servo_set()
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.

PX4/Firmware,acfloria/Firmware,acfloria/Firmware,acfloria/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,PX4/Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,PX4/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware