avcodec/dcadec: if XLL is enabled use xll channels

Fixes outputting 14 channels instead of 7
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dcadec.c

@@ -1808,7 +1808,7 @@ FF_ENABLE_DEPRECATION_WARNINGS
             /* If downmixing to stereo, don't decode additional channels.
              * FIXME: Using the xch_disable flag for this doesn't seem right. */
             if (!s->xch_disable)
-                channels += s->xll_channels - s->xll_residual_channels;
+                channels = s->xll_channels;
         }
     }