HID: wacom: report distance for Intuos4 WL

This patch adds reporting of distance of tool to the tablet surface. Maximum
reported value is 63 (0x3F).

Signed-off-by: Przemo Firszt <przemo@firszt.eu>
Acked-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>