nvme: remove nssa from struct nvme_ctrl

The reported number of streams is not used outside the function that
gets it, so no need to stash it in the controller structure. Use a local
variable instead.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>