HID: multitouch: ensure that serial devices make no use of contact count

The serial protocol makes contact count a redondant information, and
sometimes it is not reliable (TRS-Star are in this case).
Disabling the use of contact count for these devices is thus safer.
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-multitouch.c

@@ -758,6 +758,7 @@ static void mt_post_parse_default_settings(struct mt_device *td)
 		quirks &= ~MT_QUIRK_NOT_SEEN_MEANS_UP;
 		quirks &= ~MT_QUIRK_VALID_IS_INRANGE;
 		quirks &= ~MT_QUIRK_VALID_IS_CONFIDENCE;
+		quirks &= ~MT_QUIRK_CONTACT_CNT_ACCURATE;
 	}
 
 	td->mtclass.quirks = quirks;