Input: wacom_w8001 - add multitouch slot support

Some serial wacom devices support two-finger touch. Test for this during
init and parse the touch packets accordingly. Touch packets are
processed using Protocol B (MT Slots).

Note: there are several wacom versions that do touch but not two-finger
touch. These are not catered for here, touch events for these are simply
discarded.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>