libnvdimm, namespace: debug invalid interleave-set-cookie values

If platform firmware fails to populate unique / non-zero serial number
data for each nvdimm in an interleave-set it may cause pmem region
initialization to fail.  Add a debug message for this case.
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

drivers/nvdimm/namespace_devs.c

@@ -1584,8 +1584,10 @@ static int find_pmem_label_set(struct nd_region *nd_region,
 	int rc = -ENODEV, l;
 	u16 i;
 
-	if (cookie == 0)
+	if (cookie == 0) {
+		dev_dbg(&nd_region->dev, "invalid interleave-set-cookie\n");
 		return -ENXIO;
+	}
 
 	/*
 	 * Find a complete set of labels by uuid.  By definition we can start