nvme-pci: cmb sysfs: one file, one value

An attribute should only be exporting one value as recommended in
Documentation/filesystems/sysfs.rst. Implement CMB attributes this way.
The old attribute will remain for backward compatibility.

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