rtspsrc: Fix up sendonly/recvonly attribute handling

rtspsrc: Fix up sendonly/recvonly attribute handling

We can't handle recvonly streams, sendonly streams are perfectly fine.

The direction is the one from the point of view of the SDP offerer
(i.e. the RTSP server), and a recvonly stream would be one where the
server expects us to send media.

RFC 3264, section 5.1:
   If the offerer wishes to only send media on a stream to its peer, it
   MUST mark the stream as sendonly with the "a=sendonly" attribute.

This is mixed up in the ONVIF streaming specification examples, but
actual implementations and conformance tools seem to not care at all
about the attributes.

https://bugzilla.gnome.org/show_bug.cgi?id=792376