touchpad: reset motion history when nfingers changes on semi-mt pads

touchpad: reset motion history when nfingers changes on semi-mt pads

On semi-mt touchpads the reported position of the first finger down may
jump when the pad switches from st to mt mode. When this happens a large
delta gets seen on the first finger at the same time the second fingers
is first seen down, causing a spurious 2 finger scroll event.

Reset the motion history when nfingers changes on semi-mt pads to avoid this.

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