libnvdimm/labels: Fix kernel-doc for label.h

Clean up existing kernel-doc warnings before adding new CXL label data
structures.

drivers/nvdimm/label.h:66: warning: Function parameter or member 'labelsize' not described in 'nd_namespace_index'
drivers/nvdimm/label.h:66: warning: Function parameter or member 'free' not described in 'nd_namespace_index'
drivers/nvdimm/label.h:103: warning: Function parameter or member 'align' not described in 'nd_namespace_label'
drivers/nvdimm/label.h:103: warning: Function parameter or member 'reserved' not described in 'nd_namespace_label'
drivers/nvdimm/label.h:103: warning: Function parameter or member 'type_guid' not described in 'nd_namespace_label'
drivers/nvdimm/label.h:103: warning: Function parameter or member 'abstraction_guid' not described in 'nd_namespace_label'
drivers/nvdimm/label.h:103: warning: Function parameter or member 'reserved2' not described in 'nd_namespace_label'
drivers/nvdimm/label.h:103: warning: Function parameter or member 'checksum' not described in 'nd_namespace_label'

Reported-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/163116431381.2460985.6990754901097922099.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>