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>

drivers/isdn/i4l/isdn_common.c

@@ -878,6 +878,7 @@ isdn_readbchan(int di, int channel, u_char *buf, u_char *fp, int len, wait_queue
 int
 isdn_readbchan_tty(int di, int channel, struct tty_struct *tty, int cisco_hack)
 {
+	struct tty_port *port = tty->port;
 	int count;
 	int count_pull;
 	int count_put;
@@ -891,7 +892,7 @@ isdn_readbchan_tty(int di, int channel, struct tty_struct *tty, int cisco_hack)
 	if (skb_queue_empty(&dev->drv[di]->rpqueue[channel]))
 		return 0;
 
-	len = tty_buffer_request_room(tty, dev->drv[di]->rcvcount[channel]);
+	len = tty_buffer_request_room(port, dev->drv[di]->rcvcount[channel]);
 	if (len == 0)
 		return len;