core: platform: add warning if driver has no owner

Commit 9447057e

 ("platform_device: use a macro instead of
platform_driver_register") introduced a codepath which could result into
drivers having no owner. This went unnoticed for months, so add a
warning in case this happens again somewhere else somewhen.

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>