Input: alps - fix finger jumps on lifting 2 fingers on v7 touchpad

On v7 touchpads sometimes when 2 fingers are moved down on the touchpad
until they "fall of" the touchpad, the second touch will report 0 for y
(max y really since the y axis is inverted) and max x as coordinates,
rather then reporting 0, 0 as is expected for a non touching finger.

This commit detects this and treats these touches as non touching.

See the evemu-recording here:
https://bugzilla.redhat.com/attachment.cgi?id=1025058

BugLink: https://bugzilla.redhat.com/show_bug.cgi?id=1221200
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/mouse/alps.c

@@ -941,6 +941,11 @@ static void alps_get_finger_coordinate_v7(struct input_mt_pos *mt,
 	case V7_PACKET_ID_TWO:
 		mt[1].x &= ~0x000F;
 		mt[1].y |= 0x000F;
+		/* Detect false-postive touches where x & y report max value */
+		if (mt[1].y == 0x7ff && mt[1].x == 0xff0) {
+			mt[1].x = 0;
+			/* y gets set to 0 at the end of this function */
+		}
 		break;
 
 	case V7_PACKET_ID_MULTI: