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