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.mountain
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>