nvme: let namespace probing continue for unsupported features

Instead of failing to scan the namespace entirely when unsupported
features are detected, just mark the gendisk hidden but allow other
access like the upcoming per-namespace character device.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Javier González <javier.gonz@samsung.com>