Updating LED state requires access to regmap and therefore we may sleep,
so we could not do that directly form set_brightness() method.
Historically we used private work to adjust the brightness, but with the
introduction of set_brightness_blocking() we no longer need it.

As a bonus, not having our own work item means we do not have
use-after-free issue as we neglected to cancel outstanding work on
driver unbind.
Reported-by: Sven Van Asbroeck <thesven73@gmail.com>
Reviewed-by: Sven Van Asbroeck <TheSven73@googlemail.com>
Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>