libnvdimm, label: Fix sparse warning

The kbuild robot reports:

drivers/nvdimm/label.c:500:32: warning: restricted __le32 degrades to integer

...read 'nslot' into a local u32.

Reported-by: kbuild test robot <lkp@intel.com>
Acked-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>