mac802154: llsec: fix device deletion from list

This patch adds a missing list_del when a device description will be
deleted.

Cc: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>