HID: wacom: leds: handle Cintiq 24HD leds buttons

The 24HD has 1 button per LED (first three buttons of each group).
We need a special treatment for them as it's not a uniq button that
switches between the LEDs.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>