driver core: platform: document registration-failure requirement

Add an explicit comment to document that the reference initialised by
platform_device_register() needs to be released by a call to
platform_device_put() also when registration fails (cf.
device_register()).

Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20211222104213.5673-1-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>