HID: wacom: Ignore contacts in excess of declared contact count

The reports sent from some touch devices (e.g. the Cintiq 24HDT) contain
junk data in the contact slots which follow the final "valid" contact.
To avoid forwarding it to usrspace, we store the reported contact count
during the pre-process phase and then only process that many contacts.
If a device sends its contacts across multiple reports (what Microsoft
refers to as "hybrid" mode) then the contact count will be zero for
reports other than the first.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>