gst/audioconvert/gstaudioconvert.c: We don't support 64 bit integer audio, so don't try to claim we can.

Original commit message from CVS:
* gst/audioconvert/gstaudioconvert.c: (make_lossless_changes),
(gst_audio_convert_transform_caps):
We don't support 64 bit integer audio, so don't try to claim we can.
Stops us producing caps don't match our template caps.
Update comments.