HID: multitouch: require Finger field to mark Win8 reports as MT

This effectively changes collection_is_mt from
  contact ID in report->field
to
  (device is Win8 => collection is finger) && contact ID in report->field

Some devices erroneously report Pen for fingers, and Win8 stylus-on-touchscreen
devices report contact ID, but mark the accompanying touchscreen device's
collection correctly

Cc: stable@vger.kernel.org
Signed-off-by: NAhelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Acked-by: NBenjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

drivers/hid/hid-multitouch.c

@@ -604,9 +604,13 @@ static struct mt_report_data *mt_allocate_report_data(struct mt_device *td,
 		if (!(HID_MAIN_ITEM_VARIABLE & field->flags))
 			continue;
 
-		for (n = 0; n < field->report_count; n++) {
-			if (field->usage[n].hid == HID_DG_CONTACTID)
-				rdata->is_mt_collection = true;
+		if (field->logical == HID_DG_FINGER || td->hdev->group != HID_GROUP_MULTITOUCH_WIN_8) {
+			for (n = 0; n < field->report_count; n++) {
+				if (field->usage[n].hid == HID_DG_CONTACTID) {
+					rdata->is_mt_collection = true;
+					break;
+				}
+			}
 		}
 	}