firmware: vpd: do not leak kobjects

kobject_del() only unlinks kobject, we need to use kobject_put() to
make sure kobject will go away completely.

Fixes: 049a59db ("firmware: Google VPD sysfs driver")
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>