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: NPrzemo Firszt <przemo@firszt.eu>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

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: NJason Gerecke <killertofu@gmail.com>
Reviewed-by: NPing Cheng <pingc@wacom.com>
Signed-off-by: NDmitry Torokhov <dtor@mail.ru>

Cintiq 21ux2 has two sets of four LEDs on right and left side of
the tablet, respectively.
Reviewed-by: NEduard Hasenleithner <eduard@hasenleithner.at>
Tested-by: NEduard Hasenleithner <eduard@hasenleithner.at>
Signed-off-by: NPing Cheng <pingc@wacom.com>
Signed-off-by: NDmitry Torokhov <dtor@mail.ru>

The LED also indicates the status of the tablet. Don't turn it off.
Reviewed-by: NEduard Hasenleithner <eduard@hasenleithner.at>
Tested-by: NEduard Hasenleithner <eduard@hasenleithner.at>
Signed-off-by: NPing Cheng <pingc@wacom.com>
Signed-off-by: NDmitry Torokhov <dtor@mail.ru>

The LED luminance level is normally lower when no button is pressed.
Reviewed-by: NEduard Hasenleithner <eduard@hasenleithner.at>
Tested-by: NEduard Hasenleithner <eduard@hasenleithner.at>
Signed-off-by: NPing Cheng <pingc@wacom.com>
Signed-off-by: NDmitry Torokhov <dtor@mail.ru>

This commit enables control of the LEDs and OLED displays found on the
Wacom Intuos4 M, L, and XL. For this purpose, a new "wacom_led" attribute
group is added to the sysfs entry of the USB device.

This "wacom_led" group only shows up when the correct device (M, L, or XL)
is detected. The attributes are described in
 Documentation/ABI/testing/sysfs-wacom
Signed-off-by: NEduard Hasenleithner <eduard@hasenleithner.at>
Signed-off-by: NDmitry Torokhov <dtor@mail.ru>