Change protocol_mask into protocol, since we always just try a single one per

Change protocol_mask into protocol, since we always just try a single one per
iteration in make_setup_request(), and cycling between the different protocols
is now done in the calling function, therefore the need for a mask goes away.
This also makes the function somewhat simpler to read.

Discussed and approved in "[PATCH] RTSP alternate protocol 3/4".

Originally committed as revision 12505 to svn://svn.ffmpeg.org/ffmpeg/trunk