dcadec: treat all 2-channel modes as Stereo in dca_downmix.

The check for (prim_channels > 2) before calling dca_downmix made these
cases unreachable, but now 2.1 layouts will go through the downmix code.
Having dual mono, Lt/Rt and sum-difference layouts print errors when
regular Stereo doesn't seems pointless.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/dcadec.c

@@ -1001,13 +1001,13 @@ static void dca_downmix(float **samples, int srcfmt, int lfe_present,
 
     switch (srcfmt) {
     case DCA_MONO:
-    case DCA_CHANNEL:
-    case DCA_STEREO_TOTAL:
-    case DCA_STEREO_SUMDIFF:
     case DCA_4F2R:
         av_log(NULL, 0, "Not implemented!\n");
         break;
+    case DCA_CHANNEL:
     case DCA_STEREO:
+    case DCA_STEREO_TOTAL:
+    case DCA_STEREO_SUMDIFF:
         break;
     case DCA_3F:
         c = channel_mapping[0];