Only try to parse data as coming from trackpoint if firmware told us that
trackpoint is present.

Fixes commit caeb0d37Reported-and-tested-by: NMarcus Overhagen <marcus.overhagen@gmail.com>
Reported-and-tested-by: NAnders Kaseorg <andersk@mit.edu>
Signed-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>