ieee802154: 6lowpan: make header_ops const

Make this const as it is only stored as a reference in a const field of
a net_device structure.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Acked-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>