6lowpan: cleanup lowpan_header_compress

This patch changes the lowpan_header_compress function by removing
unused parameters like "len" and drop static value parameters of
protocol type. Instead we really check the protocol type inside inside
the skb structure. Also we drop the use of IEEE802154_ADDR_LEN which is
link-layer specific. Instead we using EUI64_ADDR_LEN which should always
the default case for now.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>