ffmpeg: Disable mips32r2 for mips32 CPUs

Some CPUs, such as the au1500 are MIPS32 but not MIPS32r2 and we have to
explicitly disable mips32r2 in the ffmpeg configure script.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Merge pull request #428 from br101/master

ffmpeg: Disable mips32r2 for mips32 CPUs