nfit: Don't check _STA on NVDIMM devices

The _STA only applies to the root device, not the individual NVDIMMS,
so don't check here. NVDIMM device state flags are checked elsewhere.

Signed-off-by: Linda Knippers <linda.knippers@hp.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>