TTY: switch tty_buffer_request_room to tty_port

Now, we start converting tty buffer functions to actually use
tty_port. This will allow us to get rid of the need of tty pointer in
many call sites. Only tty_port will be needed and hence no more
tty_port_tty_get calls in those paths.

Here we start with tty_buffer_request_room.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/tty/serial/pch_uart.c b/drivers/tty/serial/pch_uart.c
index 8318925..4f1774b 100644
--- a/drivers/tty/serial/pch_uart.c
+++ b/drivers/tty/serial/pch_uart.c
@@ -629,15 +629,16 @@
 	struct tty_struct *tty;
 	int room;
 	struct uart_port *port = &priv->port;
+	struct tty_port *tport = &port->state->port;
 
 	port = &priv->port;
-	tty = tty_port_tty_get(&port->state->port);
+	tty = tty_port_tty_get(tport);
 	if (!tty) {
 		dev_dbg(priv->port.dev, "%s:tty is busy now", __func__);
 		return 0;
 	}
 
-	room = tty_buffer_request_room(tty, size);
+	room = tty_buffer_request_room(tport, size);
 
 	if (room < size)
 		dev_warn(port->dev, "Rx overrun: dropping %u bytes\n",