HID: wacom: Use "Confidence" flag to prevent reporting invalid contacts

The HID descriptor of many of Wacom's touch input devices include a
"Confidence" usage that signals if a particular touch collection contains
useful data. The driver does not look at this flag, however, which causes
even invalid contacts to be reported to userspace. A lucky combination of
kernel event filtering and device behavior (specifically: contact ID 0 ==
invalid, contact ID >0 == valid; and order all data so that all valid
contacts are reported before any invalid contacts) spare most devices from
any visibly-bad behavior.

The DTH-2452 is one example of an unlucky device that misbehaves. It uses
ID 0 for both the first valid contact and all invalid contacts. Because
we report both the valid and invalid contacts, the kernel reports that
contact 0 first goes down (valid) and then goes up (invalid) in every
report. This causes ~100 clicks per second simply by touching the screen.

This patch inroduces new `confidence` flag in our `hid_data` structure.
The value is initially set to `true` at the start of a report and can be
set to `false` if an invalid touch usage is seen.

Link: https://github.com/linuxwacom/input-wacom/issues/270
Fixes: f8b6a74719b5 ("HID: wacom: generic: Support multiple tools per report")
Signed-off-by: Jason Gerecke <[e-mail]>
Tested-by: Joshua Dickens <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Jiri Kosina <[e-mail]>

drivers/hid/wacom_wac.c

···
 		return;
 
 	switch (equivalent_usage) {
+	case HID_DG_CONFIDENCE:
+		wacom_wac->hid_data.confidence = value;
+		break;
 	case HID_GD_X:
 		wacom_wac->hid_data.x = value;
 		break;
···
 	}
 
 	if (usage->usage_index + 1 == field->report_count) {
-		if (equivalent_usage == wacom_wac->hid_data.last_slot_field)
+		if (equivalent_usage == wacom_wac->hid_data.last_slot_field &&
+		    wacom_wac->hid_data.confidence)
 			wacom_wac_finger_slot(wacom_wac, wacom_wac->touch_input);
 	}
 }
···
 		return;
 
 	wacom_wac->is_invalid_bt_frame = false;
+
+	hid_data->confidence = true;
 
 	for (i = 0; i < report->maxfield; i++) {
 		struct hid_field *field = report->field[i];

drivers/hid/wacom_wac.h

···
 	bool barrelswitch;
 	bool barrelswitch2;
 	bool serialhi;
+	bool confidence;
 	int x;
 	int y;
 	int pressure;