HAL_PX4: fixed PWM output for FMUv4

libraries/AP_HAL_PX4/RCOutput.cpp

@@ -45,11 +45,13 @@ void PX4RCOutput::init(void* unused)
         _outputs[i].pwm_sub = orb_subscribe_multi(ORB_ID(actuator_outputs), i);
     }
 
+#if !defined(CONFIG_ARCH_BOARD_PX4FMU_V4)
     _alt_fd = open("/dev/px4fmu", O_RDWR);
     if (_alt_fd == -1) {
         hal.console->printf("RCOutput: failed to open /dev/px4fmu");
         return;
     }
+#endif
 
     // ensure not to write zeros to disabled channels
     _enabled_channels = 0;