HID: wacom: Add LED selector control for Wacom Intuos4 WL

Add sysfs attribute to control LED selector on Wacom Intuos4. There are 4
different LEDs on the tablet and they can be turned on by something like:

echo 50 > /sys/class/leds/(device # here)\:selector\:1/brightness

Only one can be lit at a time. The brightness range is 0 to 127. This patch
also contains short ABI description.

Signed-off-by: Przemo Firszt <przemo@firszt.eu>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/drivers/hid/Kconfig b/drivers/hid/Kconfig
index 4ecc256..6e0d65a 100644
--- a/drivers/hid/Kconfig
+++ b/drivers/hid/Kconfig
@@ -596,6 +596,7 @@
 	tristate "Wacom Bluetooth devices support"
 	depends on BT_HIDP
 	select POWER_SUPPLY
+	select LEDS_CLASS
 	---help---
 	Support for Wacom Graphire Bluetooth and Intuos4 WL tablets.