Fix tty speed handling on 8250

We try and write the correct speed back but the serial midlayer already
mangles the speed on us and that means if we request B0 we report back B9600
when we should not.  For now we'll hack around this in the drivers and serial
code, pending a better long term solution.
Signed-off-by: NAlan Cox <alan@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/serial/8250.c

@@ -2228,7 +2228,9 @@ serial8250_set_termios(struct uart_port *port, struct ktermios *termios,
 	}
 	serial8250_set_mctrl(&up->port, up->port.mctrl);
 	spin_unlock_irqrestore(&up->port.lock, flags);
-	tty_termios_encode_baud_rate(termios, baud, baud);
+	/* Don't rewrite B0 */
+	if (tty_termios_baud_rate(termios))
+		tty_termios_encode_baud_rate(termios, baud, baud);
 }
 
 static void