serial: sunsu: 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/20221019091151.6692-39-ilpo.jarvinen@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sunsu.c

@@ -417,8 +417,7 @@ static void transmit_chars(struct uart_sunsu_port *up)
 	count = up->port.fifosize;
 	do {
 		serial_out(up, UART_TX, xmit->buf[xmit->tail]);
-		xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
-		up->port.icount.tx++;
+		uart_xmit_advance(&up->port, 1);
 		if (uart_circ_empty(xmit))
 			break;
 	} while (--count > 0);