nvme-pci: use attribute group for cmb sysfs

Appending sysfs files to the controller kobject is a bit clunky and
becomes a maintenance problem as more attributes are added. The
attribute group infrastructure handles this better, so use that.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>