NVMe: removed unused nn var from nvme_dev_add

The logic in nvme_dev_add to enumerate namespaces was moved to
nvme_dev_scan. When moved, the nn variable is no longer used. This patch
removes it.

Fixes: a5768aai ("NVMe: Automatic namespace rescan")
Signed-off-by: Matias Bjørling <m@bjorling.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>