HID: hid-multitouch: Fix contact count on 3M panels

Some devices report the number of contacts via the unreliable
CONTACTCOUNT usage, rather than using the CONTACTMAX feature.
Without this patch, the 3M devices are constrained to the default
maximum of ten fingers.

Cc: Benjamin Tissoires <benjamin.tissoires@enac.fr>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>