libusb20_tr_set_length
libusb20_tr_set_length(pxfer, 0, 0);
libusb20_tr_set_length(pxfer, 8, 0);
libusb20_tr_set_length(pxfer, max_bulk, 1);
void libusb20_tr_set_length(struct libusb20_transfer *xfer, uint32_t length, uint16_t fr_index);