fix: enable Heltec V4 USB serial

boards/heltec_v4.json

@@ -9,7 +9,7 @@
     "extra_flags": [
       "-DBOARD_HAS_PSRAM",
       "-DARDUINO_USB_CDC_ON_BOOT=1",
-      "-DARDUINO_USB_MODE=0",
+      "-DARDUINO_USB_MODE=1",
       "-DARDUINO_RUNNING_CORE=1",
       "-DARDUINO_EVENT_RUNNING_CORE=1"
     ],
@@ -40,4 +40,4 @@
   },
   "url": "https://heltec.org/",
   "vendor": "heltec"
-}
+}