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/serial/ifx6x60.c

@@ -615,7 +615,7 @@ static int ifx_port_activate(struct tty_port *port, struct tty_struct *tty)
 	tty->driver_data = ifx_dev;
 
 	/* allows flip string push from int context */
-	tty->low_latency = 1;
+	port->low_latency = 1;
 
 	/* set flag to allows data transfer */
 	set_bit(IFX_SPI_STATE_IO_AVAILABLE, &ifx_dev->flags);