avformat/flvdec: move set bit_rate from parse AMF OBJECT to

 create_stream

before patch:
Stream #0:0: Video: h264 (High), yuv420p(progressive), 1280x714 [SAR 1:1 DAR 640:357], 25 fps, 25 tbr, 1k tbn, 50 tbc

after patch:
Stream #0:0: Video: h264 (High), yuv420p(progressive), 1280x714 [SAR 1:1 DAR 640:357], 2576 kb/s, 25 fps, 25 tbr, 1k tbn, 50 tbc

Signed-off-by: Steven Liu <lq@chinaffmpeg.org>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>