USB: ftdi_sio: fix TIOCSSERIAL baud_base handling

Return EINVAL if new baud_base does not match the current one.

The baud_base is device specific and can not be changed. This restores
the old (pre-2005) behaviour which was changed due to a
misunderstanding regarding this fact (see
https://lkml.org/lkml/2005/1/20/84).
Reported-by: NTorbjörn Lofterud <torbjorn@pi.nxs.se>
Signed-off-by: NJohan Hovold <jhovold@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/ftdi_sio.c

@@ -1333,8 +1333,7 @@ static int set_serial_info(struct tty_struct *tty,
 		goto check_and_exit;
 	}
 
-	if ((new_serial.baud_base != priv->baud_base) &&
-	    (new_serial.baud_base < 9600)) {
+	if (new_serial.baud_base != priv->baud_base) {
 		mutex_unlock(&priv->cfg_lock);
 		return -EINVAL;
 	}