mac802154: declare struct ieee802154_ops as const

The ieee802154_ops structure should be never changed during runtime.
This patch declare this structure as const to avoid a runtime change.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Cc: Alan Ott <alan@signal11.us>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>