base-modem: support non-serial ports when peeking the best AT port

When we don't care about the type of AT port to use, prefer the QMI or
MBIM based ones if any available.

src/plugins/sierra/mm-broadband-bearer-sierra.c

@@ -138,7 +138,7 @@ load_connection_status (MMBaseBearer        *self,
     g_task_set_task_data (task, GUINT_TO_POINTER ((guint)profile_id), NULL);
 
     /* If no control port available, error out */
-    port = MM_IFACE_PORT_AT (mm_base_modem_peek_best_at_port (modem, NULL));
+    port = mm_base_modem_peek_best_at_port (modem, NULL);
     if (!port) {
         g_task_return_new_error (task, MM_CORE_ERROR, MM_CORE_ERROR_UNSUPPORTED,
                                  "Couldn't load connection status: no control port available");