ndtest: fix typo class_regster -> class_register

Fixes: dd6cad2d

 ("testing: nvdimm: make struct class structures constant")
Signed-off-by: Yi Zhang <yi.zhang@redhat.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/20231127040026.362729-1-yi.zhang@redhat.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>