libusb20_tr_set_total_frames
libusb20_tr_set_total_frames(pxfer, iso_packets);
libusb20_tr_set_total_frames(pxfer, 2);
libusb20_tr_set_total_frames(pxfer, 1);
void libusb20_tr_set_total_frames(struct libusb20_transfer *xfer, uint32_t nFrames);