asus-laptop: use attribute group to manage attributes

Instead of registering (and removing) every attribute individually
switch to using sysfs attribute group. This makes sure that we
properly unwind and do not try to remove non-existent attributes which
may not be safe to do in the future.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Matthew Garrett <mjg@redhat.com>