net: ieee802154: forbid monitor for del llsec devkey

This patch forbids to del llsec devkey 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/20210405003054.256017-12-aahringo@redhat.com


Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>