The protocol used by the w8001 supports status fields for tip, side
switch and eraser as well as a RDY field for proximity.

The protocol has a double usage for the f2 bit in the packet. If set,
the data is either pen + side2 button or eraser. Assume eraser if the
device comes into proximity with the f2 bit set, otherwise trigger the
side2 button. If the device comes into proximity with the f2 bit and
that bit disappears afterwards, fake proximity out for the eraser and
proximity in for the pen.
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>