HID: wacom: OLEDs control over sysfs for Intuos4

Thsi patch adds ability to control OLED micro displays on Wacom Intuos4
Wireless. The OLEDS are exposed as
/sys/class/hidraw/hidraw*/device/oled{No]_img
where No. is 0 to 7

Setting an image:

dd bs=256 if=img_file of=/sys/class/hidraw/hidraw{No}/device/oled0_img

The image has to contain 256 bytes (64x32px 1 bit). More detailed
description in Documentation/ABI/testing/sysfs-driver-wacom

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