stagefright aacenc: Fix an inline asm constraint

Output-only parameters should use =r or =&r constrains, never +r.

This avoids a warning about the variable 'result' being used
uninitialized.

This avoids loading the uninitialized value into the register,
before it is overwritten by the multiplication.

Change-Id: I0ef6179e133c35d290feb8e12bea180ecae11a05

media/libstagefright/codecs/aacenc/basic_op/oper_32b.h

@@ -63,7 +63,7 @@ __inline Word32 L_mpy_wx(Word32 L_var2, Word16 var1)
 	Word32 result;
 	asm volatile(
 		"SMULWB  %[result], %[L_var2], %[var1] \n"
-		:[result]"+r"(result)
+		:[result]"=r"(result)
 		:[L_var2]"r"(L_var2), [var1]"r"(var1)
 		);
 	return result;