nl802154: Missing return in nl802154_add_llsec_key()

There was a missing return here so it meant that often
ieee802154_llsec_parse_key_id() was not called.

Fixes: a26c5fd7 ('nl802154: add support for security layer')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>