Fix H.264 8x8 transform selection

Fix H.264 8x8 transform selection

It seems that it's opt parse bug, it can't process the flags which start by
digit. After change 8x8dct to dct8x8, I can set it without problem. I guess
nobody will use it since it can't work as expected, so the quick fix is change
the option name.

Patch by Limin Wang lance dot lmwang at gmail com

Originally committed as revision 8492 to svn://svn.ffmpeg.org/ffmpeg/trunk