driver core: class: Clear private pointer on registration failures

Clear the class private pointer if __class_register() fails for it, so
as to allow its users to verify that the class is usable by checking
the value of that pointer.

For consistency, clear that pointer before freeing the object pointed
to by it in class_release().

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/4463268.LvFx2qVVIh@kreacher


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