Input: synaptics - fix middle button on Lenovo 2015 products

On the X1 Carbon 3rd gen (with a 2015 broadwell cpu), the physical middle
button of the trackstick (attached to the touchpad serio device, of course)
seems to get lost.

Actually, the touchpads reports 3 extra buttons, which falls in the switch
below to the '2' case. Let's handle the case of odd numbers also, so that
the middle button finds its way back.

Cc: stable@vger.kernel.org
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>