Fix broken KISS modem build on 6 ESP32-S3 platforms

Broken out from PR #3496 which has been open for a month.

examples/kiss_modem/main.cpp

@@ -115,7 +115,10 @@ void setup() {
   delay(100);
 #if defined(ESP32) && defined(ARDUINO_USB_CDC_ON_BOOT) && ARDUINO_USB_CDC_ON_BOOT
   Serial.setTxTimeoutMs(USB_TX_TIMEOUT_MS);
+#if defined(ARDUINO_USB_MODE) && ARDUINO_USB_MODE
+  // HWCDC only; TinyUSB's USBCDC TX buffer is fixed at build time
   Serial.setTxBufferSize(USB_TX_BUFFER_SIZE);
+#endif
 #endif
   modem = new KissModem(Serial, identity, rng, radio_driver, board, sensors);
 #endif