Try to guess the default scroll buttons a bit better. Right now we default to
scroll button 0 (disabled) whenever a device doesn't have a middle button but
we might as well cast a wider net here as setting a scroll button only has a
direct effect when button scrolling is enabled.

Use the first extra button we find or fall back onto the right button if we
don't have any extra buttons.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>