mac802154: add comments for llsec issues

While doing a little test with the llsec implementation I saw these
issues. We should move decryption and encruption somewhere else,
otherwise while capturing with wireshark the mac header shows secuirty
fields but the payload is plaintext.

A complete other issue is what doing with HardMAC drivers where the
payload is always plaintext. I think we need a special handling then in
userspace. We currently doesn't support any HardMAC transceivers, so we
should fix the first issue for SoftMAC transceivers.

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