MODE_LENGTH
char sc_mode_bytes[MODE_LENGTH]; /* device mode */
.bufsize = sizeof(struct usb_device_request) + MODE_LENGTH,
USETW(req.wLength, MODE_LENGTH);
usbd_copy_in(pc, 0, sc->sc_mode_bytes, MODE_LENGTH);
usbd_xfer_set_frame_len(xfer, 1, MODE_LENGTH);