UPLCOM_N_TRANSFER
usbd_transfer_poll(sc->sc_xfer, UPLCOM_N_TRANSFER);
struct usb_xfer *sc_xfer[UPLCOM_N_TRANSFER];
static const struct usb_config uplcom_config_data[UPLCOM_N_TRANSFER] = {
UPLCOM_N_TRANSFER, sc, &sc->sc_mtx);
usbd_transfer_unsetup(sc->sc_xfer, UPLCOM_N_TRANSFER);