serial: sunplus-uart: 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/20221019091151.6692-37-ilpo.jarvinen@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sunplus-uart.c

@@ -216,9 +216,7 @@ static void transmit_chars(struct uart_port *port)
 
 	do {
 		sp_uart_put_char(port, xmit->buf[xmit->tail]);
-		xmit->tail = (xmit->tail + 1) % UART_XMIT_SIZE;
-		port->icount.tx++;
-
+		uart_xmit_advance(port, 1);
 		if (uart_circ_empty(xmit))
 			break;
 	} while (sunplus_tx_buf_not_full(port));