nvdimm: make nd_class variable static

nvdimm: make nd_class variable static

The nd_class is not used outside of drivers/nvdimm/bus.c and thus sparse
is generating the following warning. Remove this by making it static:

drivers/nvdimm/bus.c:28:14: warning: symbol 'nd_class' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Link: https://lore.kernel.org/r/20230616160628.11801-1-ben.dooks@codethink.co.uk
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>