UMIDI_RX_TRANSFER
usbd_transfer_start(chan->xfer[UMIDI_RX_TRANSFER]);
chan->xfer[UMIDI_RX_TRANSFER] == NULL) {
usbd_transfer_start(chan->xfer[UMIDI_RX_TRANSFER]);
usbd_transfer_stop(chan->xfer[UMIDI_RX_TRANSFER]);
[UMIDI_RX_TRANSFER] = {