HID: multitouch: enable palm rejection if device implements confidence usage

BugLink: http://bugs.launchpad.net/bugs/1541671

The usage Confidence is mandary to Windows Precision Touchpad devices. The
appearance of this usage is checked in hidinput_connect but the quirk
MT_QUIRK_VALID_IS_CONFIDENCE is not applied to device accordingly.
Apply this quirk and also remove quirk MT_QUIRK_ALWAYS_VALID to enable palm
rejection for the WIN 8 touchpad devices which have implemented usage
Confidence in its input reports.

Tested on Dell XPS 13 laptop.
Signed-off-by: Allen Hung <allen_hung@dell.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
(cherry picked from commit 25a84db1)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/hid/hid-multitouch.c

@@ -497,6 +497,11 @@ static int mt_touch_input_mapping(struct hid_device *hdev, struct hid_input *hi,
 			mt_store_field(usage, td, hi);
 			return 1;
 		case HID_DG_CONFIDENCE:
+			if (cls->name == MT_CLS_WIN_8 &&
+				field->application == HID_DG_TOUCHPAD) {
+				cls->quirks &= ~MT_QUIRK_ALWAYS_VALID;
+				cls->quirks |= MT_QUIRK_VALID_IS_CONFIDENCE;
+			}
 			mt_store_field(usage, td, hi);
 			return 1;
 		case HID_DG_TIPSWITCH: