Input: do not assign new tracking ID when changing tool type

We allow changing tool type (from MT_TOOL_FINGER to MT_TOOL_PALM) so we
should not be forcing new tracking ID for the slot.

Acked-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>