Input: apanel - switch to using brightness_set_blocking()

Now that LEDs core allows "blocking" flavor of "set brightness" method we
can use it and get rid of private work item. As a bonus, we are no longer
forgetting to cancel it when we unbind the driver.

Reviewed-by: Sven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>