HID: wacom: Add pad buttons reporting on Intuos4 WL

This patch adds reporting of 1 wheel button and 8 strip buttons for Intuos4 WL.
The buttons are reported as BTN_0 to BTN_9. The change of type butstate variable
is required as the old type 'char' couldn't store state of 9 buttons. The change
is not affecting Graphire tablet as it only uses first 2 bits of 'butstate'.

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