ARM: prettify MAC16() macro

Originally committed as revision 15782 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/armv4l/mathops.h

@@ -79,7 +79,8 @@ static inline av_const int64_t MAC64(int64_t d, int a, int b)
 
 /* signed 16x16 -> 32 multiply add accumulate */
 #   define MAC16(rt, ra, rb)                                            \
-        __asm__ ("smlabb %0, %2, %3, %0" : "=r" (rt) : "0" (rt), "r" (ra), "r" (rb));
+    __asm__ ("smlabb %0, %1, %2, %0" : "+r"(rt) : "r"(ra), "r"(rb));
+
 /* signed 16x16 -> 32 multiply */
 #   define MUL16(ra, rb)                                                \
         ({ int rt;                                                    \