serial: dz: Assume previous baudrate is valid

Assume previously used termios has a valid baudrate and use
it directly.
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220816115739.10928-4-ilpo.jarvinen@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/dz.c

@@ -592,9 +592,12 @@ static void dz_set_termios(struct uart_port *uport, struct ktermios *termios,
 
 	baud = uart_get_baud_rate(uport, termios, old_termios, 50, 9600);
 	bflag = dz_encode_baud_rate(baud);
-	if (bflag < 0)	{			/* Try to keep unchanged.  */
-		baud = uart_get_baud_rate(uport, old_termios, NULL, 50, 9600);
-		bflag = dz_encode_baud_rate(baud);
+	if (bflag < 0)	{
+		if (old_termios) {
+			/* Keep unchanged. */
+			baud = tty_termios_baud_rate(old_termios);
+			bflag = dz_encode_baud_rate(baud);
+		}
 		if (bflag < 0)	{		/* Resort to 9600.  */
 			baud = 9600;
 			bflag = DZ_B9600;