Input: elantech - ignore high bits in the position coordinates

In older versions of the elantech hardware/firmware those bits always
were unset, so it didn't actually matter, but newer versions seem to
use those high bits for something else, screwing up the coordinates
we report to the input layer for those devices.

Signed-off-by: Florian Ragwitz <rafl@debian.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>