HID: multitouch: ensure that serial devices make no use of contact count

The serial protocol makes contact count a redondant information, and
sometimes it is not reliable (TRS-Star are in this case).
Disabling the use of contact count for these devices is thus safer.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>