devicewidget: fix the enabling logic of the advanced options expander

The old logic disabled the expander always when a sink didn't have any
ports. That's bad if the sink has no ports but supports configuring
formats. That hasn't caused problems so far, but I have a patch for
module-null-sink that will enable format configuration support, and the
null sink has no ports, so it's affected by this issue.

This patch also changes things so that the expander is completely hidden
when it's disabled instead just setting it non-sensitive. I think that's
better (at least it saves some space).