opusdec: Try harder to negotiate the upstream channels/rate preferences

opusdec: Try harder to negotiate the upstream channels/rate preferences

It might be possible to fulfill those but not with the first caps
structure. Instead of just fixating the first caps structure, check if
the preference can be fulfilled by any of the structures as the first
step.

Without this the following pipeline negotiates to mono after the
decoder because opusenc only has a single channel in its first caps
structure.

    gst-launch-1.0 audiotestsrc ! audio/x-raw,channels=2 ! opusenc \
        ! queue ! opusdec ! queue ! opusenc ! fakesink

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3689>