HID: wacom: Use half of brightness for low state

Intuos4 WL leds have 2 states: high and low. With no activity tablet
uses low state; using pen triggers high state. Both can be set
independently, but it would require setting another led device to
control them separately. This patch uses the brightness set through leds
subsytem as high and half of that value as low.

Signed-off-by: Przemo Firszt <przemo@firszt.eu>
Acked-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>