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>
diff --git a/drivers/isdn/gigaset/interface.c b/drivers/isdn/gigaset/interface.c
index dd2a57e..6dcecd4 100644
--- a/drivers/isdn/gigaset/interface.c
+++ b/drivers/isdn/gigaset/interface.c
@@ -164,7 +164,7 @@
 
 	if (cs->port.count == 1) {
 		tty_port_tty_set(&cs->port, tty);
-		tty->low_latency = 1;
+		cs->port.low_latency = 1;
 	}
 
 	mutex_unlock(&cs->mutex);