6lowpan: add lowpan_is_ll function

This patch adds the lowpan_is_ll function, which can be used to make a
special 6lowpan linklayer handling for a specific 6lowpan linklayer
type.

Signed-off-by: Alexander Aring <aar@pengutronix.de>
Reviewed-by: Stefan <Schmidt&lt;stefan@osg.samsung.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>