Input: elantech - fix V4 report decoding for module with middle key

Input: elantech - fix V4 report decoding for module with middle key

BugLink: https://bugs.launchpad.net/bugs/1779802
Some touchpad has middle key and it will be indicated in bit 2 of packet[0].
We need to fix V4 formation's byte mask to prevent error decoding.

Signed-off-by: KT Liao <kt.liao@emc.com.tw>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
(cherry picked from commit e0ae2519ca004a628fa55aeef969c37edce522d3)
Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Acked-by: Hui Wang <hui.wang@canonical.com>
Acked-By: Wen-chien Jesse Sung <jesse.sung@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>