nvmet-auth: allow to clear DH-HMAC-CHAP keys

As we can set DH-HMAC-CHAP keys, we should also be
able to unset them.

Signed-off-by: Hannes Reinecke <hare@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>