HID: wacom: Add BTN_TOOL_FINGER for pad button reporting

Without this patch xf86-input-wacom driver wasn't able to properly recognise
pad button events. It was also causing some problems with button mapping.

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