nvme: add error handling support for add_disk()

We never checked for errors on add_disk() as this function
returned void. Now that this is fixed, use the shiny new
error handling.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>