net: ieee802154: forbid monitor for add llsec key

This patch forbids to add llsec key for monitor interfaces 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/20210405003054.256017-5-aahringo@redhat.com
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>