usb_buf: count number of elements in queue

This is in preparation for limiting the maximum queue length

Change-Id: I7cb184d7a1ccb519010a2f3e3295cc3a5fbf8052
Related: OS#4251

firmware/libcommon/source/usb_buf.c

@@ -78,7 +78,7 @@ int usb_buf_submit(struct msgb *msg)
 
 	/* no need for irqsafe operation, as the usb_tx_queue is
 	 * processed only by the main loop context */
-	msgb_enqueue(&ep->queue, msg);
+	msgb_enqueue_count(&ep->queue, msg, &ep->queue_len);
 	return 0;
 }