nvme : Use correct scnprintf in cmb show

Make sure we are using the correct scnprintf in the sysfs show
function for the CMB.

Signed-off-by: Stephen Bates <sbates@raithlin.com>
Reviewed-by Jon Derrick: <jonathan.derrick@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>