avcodec/dcadec: make dca_dmix_code() 650% faster

This effectively replaces the ldexpf() function call by a multiplication
with a constant.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dcadec.c

@@ -2107,7 +2107,7 @@ static float dca_dmix_code(unsigned code)
 {
     int sign = (code >> 8) - 1;
     code &= 0xff;
-    return ldexpf((dca_dmixtable[code] ^ sign) - sign, -15);
+    return ((dca_dmixtable[code] ^ sign) - sign) * pow(2, -15);
 }
 
 /**