Clean up example-usb-serial.

example-usb-serial/usb-cdc.c

@@ -586,8 +586,7 @@ usb_set_interface (struct usb_dev *dev)
   else
     {
       vcom_setup_endpoints_for_interface (interface, 0);
-      usb_lld_ctrl_ack (dev);
-      return 0;
+      return usb_lld_ctrl_ack (dev);
     }
 }