usb: gadget: serial: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 1350962
Signed-off-by: NGustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: NFelipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/u_serial.c

@@ -1078,6 +1078,7 @@ static void gs_complete_out(struct usb_ep *ep, struct usb_request *req)
 	default:
 		pr_warn("%s: unexpected %s status %d\n",
 			__func__, ep->name, req->status);
+		/* fall through */
 	case 0:
 		/* normal completion */
 		spin_lock(&info->con_lock);