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).