TTY: move low_latency to tty_port

One point is to have less places where we actually need tty pointer.
The other is that low_latency is bound to buffer processing and
buffers are now in tty_port. So it makes sense to move low_latency to
tty_port too.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/mxser.c

@@ -1264,7 +1264,7 @@ static int mxser_set_serial_info(struct tty_struct *tty,
 				(new_serial.flags & ASYNC_FLAGS));
 		port->close_delay = new_serial.close_delay * HZ / 100;
 		port->closing_wait = new_serial.closing_wait * HZ / 100;
-		tty->low_latency = (port->flags & ASYNC_LOW_LATENCY) ? 1 : 0;
+		port->low_latency = (port->flags & ASYNC_LOW_LATENCY) ? 1 : 0;
 		if ((port->flags & ASYNC_SPD_MASK) == ASYNC_SPD_CUST &&
 				(new_serial.baud_base != info->baud_base ||
 				new_serial.custom_divisor !=