serial: milbeaut_usio: Use uart_xmit_advance()

Take advantage of the new uart_xmit_advance() helper.
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20221019091151.6692-24-ilpo.jarvinen@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/milbeaut_usio.c

@@ -98,8 +98,7 @@ static void mlb_usio_tx_chars(struct uart_port *port)
 	do {
 		writew(xmit->buf[xmit->tail], port->membase + MLB_USIO_REG_DR);
 
-		xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
-		port->icount.tx++;
+		uart_xmit_advance(port, 1);
 		if (uart_circ_empty(xmit))
 			break;