net: ieee802154: stop dump llsec devs for monitors

This patch stops dumping llsec devs for monitors which we don't support
yet. Otherwise we will access llsec mib which isn't initialized for
monitors.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Link: https://lore.kernel.org/r/20210405003054.256017-7-aahringo@redhat.com
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>