package/ffmpeg: fix build on mips

mips_32 is not supported by ffmpeg and it tries to build with loongson3
SIMD support that leads to build failure due to:
/tmp/ccFO2LRa.s: Assembler messages:
/tmp/ccFO2LRa.s:15314: Error: opcode not supported on this processor: mips32 (mips32) `dmult $2,$6'
/tmp/ccFO2LRa.s:15316: Error: opcode not supported on this processor: mips32 (mips32) `dsrl $2,$2,32'

So let's --disable-asm to prevent using those unsupported opcodes for every
mips architecture according to Arnout.

Fixes:
http://autobuild.buildroot.net/results/f01/f01d9cedec8e1b371308d0f7af561a75883fa27c/



Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>