Input: trackpoint - assume 3 buttons when buttons detection fails

commit 293b915fd9bebf33cdc906516fb28d54649a25ac upstream.

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>
Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>