wacom: Set combo-topbutton to current value for styli with > 2 buttons

Although the Wacom panel doesn't have explicit support for styli with more
than two buttons, it tries to at least allow configuration of the upper and
lower buttons. This commit fixes an incorrect conditional which prevents
the panel from setting the combo box for the upper switch to the current
setting.

https://bugzilla.gnome.org/show_bug.cgi?id=790028