hamradio: add missing sanity check to tty operation

Add missing sanity check to tty operation.
Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Eugene Teo <eteo@redhat.com>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/net/hamradio/mkiss.c

@@ -548,7 +548,7 @@ static int ax_xmit(struct sk_buff *skb, struct net_device *dev)
 		}
 
 		printk(KERN_ERR "mkiss: %s: transmit timed out, %s?\n", dev->name,
-		       (ax->tty->ops->chars_in_buffer(ax->tty) || ax->xleft) ?
+		       (tty_chars_in_buffer(ax->tty) || ax->xleft) ?
 		       "bad line quality" : "driver error");
 
 		ax->xleft = 0;