touchpad: drop the tap finger count

touchpad: drop the tap finger count

Use tp->nfingers_down as trigger when we have no fingers left on the touchpad
and when we should return to idle. If all touchpoints end in the same frame
tp->nfingers is 0. Thus when we handle the first tap release we transition to
IDLE which now needs to handle (and discard) any touch release events.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>