6lowpan: ndisc: add missing 802.15.4 only check

This patch adds a missing check to handle short address parsing for
802.15.4 6LoWPAN only.

Signed-off-by: Alexander Aring <aar@pengutronix.de>
Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>