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 293b915f)
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>

drivers/input/mouse/trackpoint.c

@@ -380,8 +380,8 @@ int trackpoint_detect(struct psmouse *psmouse, bool set_properties)
 		return 0;
 
 	if (trackpoint_read(&psmouse->ps2dev, TP_EXT_BTN, &button_info)) {
-		psmouse_warn(psmouse, "failed to get extended button data\n");
-		button_info = 0;
+		psmouse_warn(psmouse, "failed to get extended button data, assuming 3 buttons\n");
+		button_info = 0x33;
 	}
 
 	psmouse->private = kzalloc(sizeof(struct trackpoint_data), GFP_KERNEL);