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

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>