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>
(cherry picked from commit 4e822fcadf409c3eb4e721ad6780117ef77f45d7)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/ffmpeg/ffmpeg.mk

@@ -507,10 +507,8 @@ else
 FFMPEG_CONF_OPTS += --enable-mipsfpu
 endif
 
-# Fix build failure on "addi opcode not supported"
-ifeq ($(BR2_mips_32r6)$(BR2_mips_64r6),y)
+# Fix build failure on several missing assembly instructions
 FFMPEG_CONF_OPTS += --disable-asm
-endif
 endif # MIPS
 
 ifeq ($(BR2_POWERPC_CPU_HAS_ALTIVEC),y)