class: use IS_ERR_OR_NULL() helper in class_unregister()

Use IS_ERR_OR_NULL() helper in class_unregister() to simplify code.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20220822061922.3884113-1-yangyingliang@huawei.com


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