nvmet: make 'tsas' attribute idempotent for RDMA

The RDMA transport defines values for TSAS, but it cannot be changed as
we only support the 'connected' mode.
So to avoid errors during reconfiguration we should allow to write the
current value.

Fixes: 3f123494 ("nvmet: make TCP sectype settable via configfs")
Signed-off-by: Hannes Reinecke <hare@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>