USBSER_DS_FIFO_DRAIN
rval = USBSER_DS_FIFO_DRAIN(pp, USBSER_TX_FIFO_DRAIN_TIMEOUT);
(void) USBSER_DS_FIFO_DRAIN(pp, DS_TX);
(void) USBSER_DS_FIFO_DRAIN(pp, USBSER_TX_FIFO_DRAIN_TIMEOUT);