because the flv specification said the video frametype
should use value range from 0x00 to 0x70,
so use 0xF0 have no problem before support enhanced flv,
but the 0xF0 will get incorrect result after support enhanced flv,
so should set the video frametype mask 0x70 to make it correct now.
Reported-By: flvAnalyser <hybase@qq.com>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>