driver core: test_async: fix an error code

The test_platform_device_register_node() function should return error
pointers instead of NULL.  That is what the callers are expecting.

Fixes: 57ea974f ("driver core: Rewrite test_async_driver_probe to cover serialization and NUMA affinity")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/1e11ed19-e1f6-43d8-b352-474134b7c008@moroto.mountainSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/test/test_async_driver_probe.c

@@ -84,7 +84,7 @@ test_platform_device_register_node(char *name, int id, int nid)
 
 	pdev = platform_device_alloc(name, id);
 	if (!pdev)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	if (nid != NUMA_NO_NODE)
 		set_dev_node(&pdev->dev, nid);