AP_BattMonitor: fix warning in UAVCAN driver

../../libraries/AP_BattMonitor/AP_BattMonitor_UAVCAN.cpp: In member function ‘virtual void AP_BattMonitor_UAVCAN::init()’:
../../libraries/AP_BattMonitor/AP_BattMonitor_UAVCAN.cpp:15:123: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘AP_Int32 {aka AP_ParamT<int, (ap_var_type)3u>}’ [-Wformat=]
 #define debug_bm_uavcan(level, fmt, args...) do { if ((level) <= AP_BoardConfig_CAN::get_can_debug()) { printf(fmt, ##args); }} while (0)
                                                                                                                           ^
../../libraries/AP_BattMonitor/AP_BattMonitor_UAVCAN.cpp:36:33: note: in expansion of macro ‘debug_bm_uavcan’
                                 debug_bm_uavcan(2, "UAVCAN BattMonitor BatteryInfo registered id: %d\n\r", _params._serial_number);

libraries/AP_BattMonitor/AP_BattMonitor_UAVCAN.cpp

@@ -38,7 +38,7 @@ void AP_BattMonitor_UAVCAN::init()
         switch (_type) {
             case UAVCAN_BATTERY_INFO:
                 if (ap_uavcan->register_BM_bi_listener_to_id(this, _params._serial_number)) {
-                    debug_bm_uavcan(2, "UAVCAN BattMonitor BatteryInfo registered id: %d\n\r", _params._serial_number);
+                    debug_bm_uavcan(2, "UAVCAN BattMonitor BatteryInfo registered id: %d\n\r", _params._serial_number.get());
                 }
                 break;
         }