fix GST_FF_AUD_CAPS_NEW to not include channels and rate in unfixed caps and make audio caps use them again. This ens...

Original commit message from CVS:
fix GST_FF_AUD_CAPS_NEW to not include channels and rate in unfixed caps and make audio caps use them again. This ensures that audio encoders set these properties when setting fixed caps and audio decoders can advertise fixed caps, too.  This should work in all cases. If not, change it back and ffdec_mp3 is broken again ;)