serial: 8250_omap: Convert to use uart_xmit_advance()

uart_xmit_advance() provides a common way on how to advance
the Tx queue. Use it for the sake of unification and robustness.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220909091258.68886-1-andriy.shevchenko@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_omap.c

@@ -984,9 +984,7 @@ static void omap_8250_dma_tx_complete(void *param)
 
 	dma->tx_running = 0;
 
-	xmit->tail += dma->tx_size;
-	xmit->tail &= UART_XMIT_SIZE - 1;
-	p->port.icount.tx += dma->tx_size;
+	uart_xmit_advance(&p->port, dma->tx_size);
 
 	if (priv->delayed_restore) {
 		priv->delayed_restore = 0;