HAL_PX4: disable manual override in PX4 for now

libraries/AP_HAL_PX4/RCOutput.cpp

@@ -23,7 +23,10 @@ void PX4RCOutput::init(void* unused)
     if (_pwm_fd == -1) {
         hal.scheduler->panic("Unable to open " PWM_OUTPUT_DEVICE_PATH);
     }
-    ioctl(_pwm_fd, PWM_SERVO_ARM, 0);
+    if (ioctl(_pwm_fd, PWM_IO_SET_FEATURES, PWM_IO_FEATURE_ARM_OK) != 0 ||
+        ioctl(_pwm_fd, PWM_SERVO_ARM, 0) != 0) {
+        hal.console->printf("RCOutput: Unable to setup IO arming\n");
+    }
 }
 
 void PX4RCOutput::set_freq(uint32_t chmask, uint16_t freq_hz)