serial: 8250_mtk: Fix uart_get_baud_rate warning

Mediatek 8250 port supports speed higher than uartclk / 16. If the baud
rates in both the new and the old termios setting are higher than
uartclk / 16, the WARN_ON in uart_get_baud_rate() will be triggered.
Passing NULL as the old termios so uart_get_baud_rate() will use
uartclk / 16 - 1 as the new baud rate which will be replaced by the
original baud rate later by tty_termios_encode_baud_rate() in
mtk8250_set_termios().

Fixes: 551e553f ("serial: 8250_mtk: Fix high-speed baud rates clamping")
Signed-off-by: Claire Chang <tientzu@chromium.org>
Link: https://lore.kernel.org/r/20201102120749.374458-1-tientzu@chromium.org
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_mtk.c

@@ -317,7 +317,7 @@ mtk8250_set_termios(struct uart_port *port, struct ktermios *termios,
 	 */
 	baud = tty_termios_baud_rate(termios);
 
-	serial8250_do_set_termios(port, termios, old);
+	serial8250_do_set_termios(port, termios, NULL);
 
 	tty_termios_encode_baud_rate(termios, baud, baud);