avformat/flv: correct the video frametype mask to 0x70

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>

libavformat/flv.h

@@ -48,7 +48,7 @@
 #define FLV_AUDIO_CODECID_MASK    0xf0
 
 #define FLV_VIDEO_CODECID_MASK    0x0f
-#define FLV_VIDEO_FRAMETYPE_MASK  0xf0
+#define FLV_VIDEO_FRAMETYPE_MASK  0x70
 
 #define AMF_END_OF_OBJECT         0x09