tty: Fix hvc return

HVC returns a size of -1 bytes for the write room in some cases.
This is bogus and not handled by the tty layer at all.
Signed-off-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/hvc/hvc_console.c

@@ -558,7 +558,7 @@ static int hvc_write_room(struct tty_struct *tty)
 	struct hvc_struct *hp = tty->driver_data;
 
 	if (!hp)
-		return -1;
+		return 0;
 
 	return hp->outbuf_size - hp->n_outbuf;
 }