driver core: test_async: fix up typo found by 0-day

0-day pointed out a typo in the platform device registration logic, so
fix it.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Guenter Roeck <groeck@chromium.org>
Cc: Dmitry Torokhov <dtor@chromium.org>
Cc: Julia Lawall <julia.lawall@lip6.fr>
Cc: Thierry Escande <thierry.escande@collabora.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>