nvdimm: fix typo in label-size definition

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Fixes: commit da6789c2 ("nvdimm: add a macro for property "label-size"")
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Cc: Haozhong Zhang <haozhong.zhang@intel.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>