slave.echo correct size

Core/Src/NbusSlave.cpp

@@ -61,7 +61,7 @@ DataFrame *NbusSlave::nbus_echo(uint8_t *echo_data, uint8_t size)
     }
 
     _sensor_count = 0;
-    return _communicator->sendAndReceive(&_pdu, _sensor_cache, 4);
+    return _communicator->sendAndReceive(&_pdu, _sensor_cache, size);
 }
 
 uint8_t NbusSlave::nbus_get_sensor_count(bool check_hw)