NVMe: Fix nvme module init when nvme_major is set

register_blkdev returns 0 when given a valid major number.

Reported-by: Ross Zwisler <ross.zwisler@intel.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>