Input: elantech - enable middle button of touchpads on ThinkPad P52

PNPID is better way to identify the type of touchpads.
Enable middle button support on 2 types of touchpads on Lenovo P52.

Cc: stable@vger.kernel.org
Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>