nvmet: do not return 'reserved' for empty TSAS values

The 'TSAS' value is only defined for TCP and RDMA, but returning
'reserved' for undefined values tricked nvmetcli to try to write
'reserved' when restoring from a config file. This caused an error
and the configuration would not be applied.

Fixes: 3f123494

 ("nvmet: make TCP sectype settable via configfs")
Signed-off-by: Hannes Reinecke <hare@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>