6lowpan: iphc: override l2 packet information

The skb->pkt_type need to be set by L2, but on 6LoWPAN there exists L2
e.g. BTLE which doesn't has multicast addressing. If it's a multicast or
not is detected by IPHC headers multicast bit. The IPv6 layer will
evaluate this pkt_type, so we force set this type while uncompressing.
Should be okay for 802.15.4 as well.

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