Input: trackpoint - assume 3 buttons when buttons detection fails

Input: trackpoint - assume 3 buttons when buttons detection fails

BugLink: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1715271
Trackpoint buttons detection fails on ThinkPad 570 and 470 series,
this makes the middle button of the trackpoint to not being recogized.
As I don't believe there is any trackpoint with less than 3 buttons this
patch just assumes three buttons when the extended button information
read fails.

Signed-off-by: Oscar Campos <oscar.campos@member.fsf.org>
Acked-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
(backported from commit 293b915fd9bebf33cdc906516fb28d54649a25ac)
Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Hui Wang <hui.wang@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>