Input: elantech - adjust hw_version detection logic

This patch fixes some v3 hardware (fw_version: 0x150500) wrongly detected
as v2 hardware.

Reported-by: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: JJ Ding <jj_ding@emc.com.tw>
Tested-By: Marc Dietrich <marvin24@gmx.de>
Acked-by: Éric Piel <eric.piel@tremplin-utc.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>