rtsp-media: Get rates only on sender streams

When play a media with both sender and receiver stream, like ONVIF back channel audio in, gst_rtsp_media_get_rates call gst_rtsp_stream_get_rates for each stream to set the rates. But gst_rtsp_stream_get_rates return false for the receiver steam, which lead a g_assert crash.

Instead to get rates on all streams, now just get rates on sender streams.