HID: wacom: Correct behavior when processing some confidence == false touches

There appear to be a few different ways that Wacom devices can deal with
confidence:

  1. If the device looses confidence in a touch, it will first clear
     the tipswitch flag in one report, and then clear the confidence
     flag in a second report. This behavior is used by e.g. DTH-2452.

  2. If the device looses confidence in a touch, it will clear both
     the tipswitch and confidence flags within the same report. This
     behavior is used by some AES devices.

  3. If the device looses confidence in a touch, it will clear *only*
     the confidence bit. The tipswitch bit will remain set so long as
     the touch is tracked. This behavior may be used in future devices.

The driver does not currently handle situation 3 properly. Touches that
loose confidence will remain "in prox" and essentially frozen in place
until the tipswitch bit is finally cleared. Not only does this result
in userspace seeing a stuck touch, but it also prevents pen arbitration
from working properly (the pen won't send events until all touches are
up, but we don't currently process events from non-confident touches).

This commit centralizes the checking of the confidence bit in the
wacom_wac_finger_slot() function and has 'prox' depend on it. In the
case where situation 3 is encountered, the treat the touch as though
it was removed, allowing both userspace and the pen arbitration to
act normally.

Signed-off-by: Tatsunosuke Tobita <tatsunosuke.tobita@wacom.com>
Signed-off-by: Ping Cheng <ping.cheng@wacom.com>
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Fixes: 7fb0413b ("HID: wacom: Use "Confidence" flag to prevent reporting invalid contacts")
Cc: stable@vger.kernel.org
Signed-off-by: Jiri Kosina <jkosina@suse.com>