net: ieee802154: forbid monitor for del llsec seclevel

This patch forbids to del llsec seclevel for monitor interfaces which we
don't support yet. Otherwise we will access llsec mib which isn't
initialized for monitors.

Reported-by:  <syzbot+fbf4fc11a819824e027b@syzkaller.appspotmail.com>
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Link: https://lore.kernel.org/r/20210405003054.256017-15-aahringo@redhat.com


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