evdev: always default to the middle button for button-scrolling

evdev: always default to the middle button for button-scrolling

The current code only defaulted to the middle button for those devices that
used button scrolling by default, requiring the user to enable button
scrolling _and_ set the button before it is active. This causes some
confusion.

There is no real benefit to leaving the button at 0 when the scroll
method isn't enabled anyway. So always default to the middle button (if
available).

https://bugzilla.redhat.com/show_bug.cgi?id=1227182

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>