staging: android: ion_test: fix check of platform_device_register_simple() error code

On error platform_device_register_simple() returns ERR_PTR() value,
check for NULL always fails. The change corrects the check itself and
propagates the returned error upwards.

Fixes: 81fb0b90

 ("staging: android: ion_test: unregister the platform device")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>