Input: wacom - add Intuos4 LED and OLED control

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: Eduard Hasenleithner <eduard@hasenleithner.at>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>