AP_Compass: Remove comments and use correct board number

libraries/AP_Compass/AP_Compass.cpp

@@ -645,6 +645,9 @@ void Compass::_detect_backends(void)
     case AP_BoardConfig::PX4_BOARD_PIXHAWK_PRO:
         ADD_BACKEND(DRIVER_AK8963, AP_Compass_AK8963::probe_mpu9250(*this, 0, ROTATION_ROLL_180_YAW_90),
                     AP_Compass_AK8963::name, false);
+        ADD_BACKEND(DRIVER_LIS3MDL, AP_Compass_LIS3MDL::probe(*this, hal.spi->get_device(HAL_COMPASS_LIS3MDL_NAME),
+                                                              false, ROTATION_PITCH_180),
+                    AP_Compass_LIS3MDL::name, false);
         break;
 
     case AP_BoardConfig::PX4_BOARD_PHMINI: