apply the same USB descriptor's iSerialNumber logic for ESP32-S2 as t… (#7563)

* apply the same USB descriptor's iSerialNumber logic for ESP32-S2 as the ESP32-S3 already has

* response on request from code reviewer

cores/esp32/USB.cpp

@@ -160,7 +160,7 @@ ESPUSB::~ESPUSB(){
 
 bool ESPUSB::begin(){
     if(!_started){
-#if CONFIG_IDF_TARGET_ESP32S3
+#if CONFIG_IDF_TARGET_ESP32S2 || CONFIG_IDF_TARGET_ESP32S3
         if(serial_number == "__MAC__"){
             StreamString s;
             uint8_t m[6];