ieee802154: 6lowpan: fix intra pan id check

The RIOT-OS stack does send intra-pan frames but don't set the intra pan
flag inside the mac header. It seems this is valid frame addressing but
inefficient. Anyway this patch adds a new function for intra pan
addressing, doesn't matter if intra pan flag or source and destination
are the same. The newly introduction function will be used to check on
intra pan addressing for 6lowpan.

Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>