lavc/dvdec: export bitrate
Changes the result of fate-mxf-probe-dv25, where the bitrate is now
exported.
Also changes the result of fate-bsf-dv-error-marker, where the exported
bitrate is now different. Note that the codec layer bitrate does not
match the container bitrate, because container timing is 25fps, while
the DV profile is 50.