mac802154: constify ieee802154_llsec_ops structure

The ieee802154_llsec_ops structure is never modified, so declare it as
const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>