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: Dan Williams <dan.j.williams@intel.com>
diff --git a/drivers/nvdimm/namespace_devs.c b/drivers/nvdimm/namespace_devs.c
index 6b0449b..4f0a213 100644
--- a/drivers/nvdimm/namespace_devs.c
+++ b/drivers/nvdimm/namespace_devs.c
@@ -1584,8 +1584,10 @@
 	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