Input: elantech - get rid of ETP_2FT_* in elantech.h

For two finger touches the coordinate of each finger gets reported
separately but with reduced resolution.

With this change, we now have the same range for ST and MT data and
scale MT data because it has lower resolution to match ST.

Suggested-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: JJ Ding <jj_ding@emc.com.tw>
Acked-by: Daniel Kurtz <djkurtz@chromium.org>
Acked-by: Éric Piel <eric.piel@tremplin-utc.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>