HID: wacom: Ignore the confidence flag when a touch is removed

AES hardware may internally re-classify a contact that it thought was
intentional as a palm. Intentional contacts are reported as "down" with
the confidence bit set. When this re-classification occurs, however, the
state transitions to "up" with the confidence bit cleared. This kind of
transition appears to be legal according to Microsoft docs, but we do
not handle it correctly. Because the confidence bit is clear, we don't
call `wacom_wac_finger_slot` and update userspace. This causes hung
touches that confuse userspace and interfere with pen arbitration.

This commit adds a special case to ignore the confidence flag if a contact
is reported as removed. This ensures we do not leave a hung touch if one
of these re-classification events occured. Ideally we'd have some way to
also let userspace know that the touch has been re-classified as a palm
and needs to be canceled, but that's not possible right now :)

Link: https://github.com/linuxwacom/input-wacom/issues/288
Fixes: 7fb0413b

 (HID: wacom: Use "Confidence" flag to prevent reporting invalid contacts)
CC: stable@vger.kernel.org
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Ping Cheng <ping.cheng@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>