Input: wacom - fix 3rd-gen Bamboo MT when 4+ fingers are in use

The message count field uses three bits of storage, not two.

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Acked-by: Chris Bagwell <chris@cnpbagwell.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>