nvmet: Set 'TREQ' to 'required' when TLS is enabled

The current implementation does not support secure concatenation,
so 'TREQ' is always set to 'required' when TLS is enabled.

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