6lowpan: iphc: fix stateful multicast compression

In case of multicast address we need to set always the LOWPAN_IPHC_M bit
and if a destination context identifier was found for a multicast
address then we need to set the LOWPAN_IPHC_DAC as well.

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