serial: mvebu-uart: Use uart_xmit_advance()

Take advantage of the new uart_xmit_advance() helper.
Reviewed-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20221019091151.6692-25-ilpo.jarvinen@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/mvebu-uart.c

@@ -223,8 +223,7 @@ static void mvebu_uart_start_tx(struct uart_port *port)
 
 	if (IS_EXTENDED(port) && !uart_circ_empty(xmit)) {
 		writel(xmit->buf[xmit->tail], port->membase + UART_TSH(port));
-		xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
-		port->icount.tx++;
+		uart_xmit_advance(port, 1);
 	}
 
 	ctl = readl(port->membase + UART_INTR(port));