HID: wacom: rely on actual touch down count to decide touch_down

touch_down is a flag to indicate if there are touches on tablet
or not. Since one set of touch events may be posted over more
than one data packet/touch frame, and pen may come in proximity
while touch events are partially sent, counting all touch events
for the set reflects the actual status of touch_down.

Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>