Input: qt2160 - 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 items.

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