v4l2: fix regression that caused ffmpeg to occasionally get stuck

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavdevice/v4l2.c

@@ -808,6 +808,7 @@ static int v4l2_set_parameters(AVFormatContext *s1)
     }
     s1->streams[0]->avg_frame_rate.num = tpf->denominator;
     s1->streams[0]->avg_frame_rate.den = tpf->numerator;
+    s1->streams[0]->r_frame_rate = s1->streams[0]->avg_frame_rate;
 
     return 0;
 }