USB: visor: fix trivial accounting bug in visor driver

usb:usbserial:visor: fix accounting in error case

data not pushed to the tty layer due to an error mustn't be counted
Signed-off-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/visor.c

@@ -513,6 +513,7 @@ static void visor_read_bulk_callback(struct urb *urb)
 			tty_kref_put(tty);
 		}
 		spin_lock(&priv->lock);
+		if (tty)
 			priv->bytes_in += available_room;
 
 	} else {