fmu: don't disarm with disarmed set

If disarmed PWM values have been set, we can't accept a disarm command.
If we did, the PWM output stop alltogether after the high pulse of the
ESC calibration.

src/drivers/px4fmu/fmu.cpp

@@ -1533,7 +1533,12 @@ PX4FMU::pwm_ioctl(file *filp, int cmd, unsigned long arg)
 		break;
 
 	case PWM_SERVO_DISARM:
-		update_pwm_out_state(false);
+
+		/* Ignore disarm if disarmed PWM is set already. */
+		if (_num_disarmed_set == 0) {
+			update_pwm_out_state(false);
+		}
+
 		break;
 
 	case PWM_SERVO_GET_DEFAULT_UPDATE_RATE: