Input: elantech - trust firmware about trackpoint presence

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

Fixes commit caeb0d37

Reported-and-tested-by: Marcus Overhagen <marcus.overhagen@gmail.com>
Reported-and-tested-by: Anders Kaseorg <andersk@mit.edu>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>