UMODEM_BULK_RD
[UMODEM_BULK_RD] = {
usbd_transfer_start(sc->sc_xfer[UMODEM_BULK_RD]);
usbd_transfer_stop(sc->sc_xfer[UMODEM_BULK_RD]);
usbd_xfer_set_stall(sc->sc_xfer[UMODEM_BULK_RD]);