Input: elantech - detect middle button based on firmware version

Looks like the new generation of Lenovo machine also need to
be added to the PnPID whitelist. This is definitively not going
to scale, as there is nothing that tells us currently if a
touchpad supports a true physical middle button.

Consider that all new touchpads that are not clickpads
(so matching ETP_NEW_IC_SMBUS_HOST_NOTIFY) are handling 3 physical
buttons.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>