net: 6lowpan: constify lowpan_nhc structures

This patch constify the lowpan_nhc declarations. Since we drop the rb
node datastructure there is no need for runtime manipulation of this
structure.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Reviewed-by: Stefan Schmidt <stefan@datenfreihafen.org>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Link: https://lore.kernel.org/r/20220428030534.3220410-4-aahringo@redhat.com
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>