Input: wacom - add LED support for Cintiq 24HD

The Cintiq 24HD has three LEDs on the left side of the tablet and
three LEDs on the right side of the tablet. Switching to LED 0,
1, or 2 will enable the top, middle, or bottom LED for the respective
side. Switching to LED 3 turns off the LEDs on the respective side.

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Reviewed-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>