net: ieee802154: stop dump llsec keys for monitors

This patch stops dumping llsec keys 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/20210405003054.256017-4-aahringo@redhat.com
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>