fix: send_arq_raw busy check compared a threading.Event, not the ARQ state

POST /modem/send_arq_raw returned 503 Modem Busy on every request:
is_modem_busy is a threading.Event and the object is always truthy.
Use getARQ() like the beacon, message send and CQ handler do.

Note for later readers: is_set() would be inverted, since setARQ(True)
clears the event. getARQ() == True means busy.

freedata_server/api/modem.py

@@ -453,7 +453,7 @@ async def post_arq_raw(
     """
     if not ctx.state_manager.is_modem_running:
         api_abort("Modem not running.", 503)
-    if ctx.state_manager.is_modem_busy:
+    if ctx.state_manager.getARQ():
         api_abort("Modem Busy.", 503)
     dxcall = payload.get("dxcall")
     data_type = payload.get("type")