dca: map xxch side channels to AV_CH_SIDE_LEFT/RIGHT instead of REAR.

This fixes decoding of certain 7.1 DTS-HD HRA files, as well as matches the spec better.
Table 6-22 in the DTS spec labels the two channels "Left/Right surround on side", and not rear.
Reviewed-by: Benjamin Larsson <benjamin@southpole.se>
Reviewed-by: Nick Brereton <nick@nbrereton.net>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dca.c

@@ -136,8 +136,8 @@ static const uint32_t map_xxch_to_native[28] = {
     AV_CH_BACK_CENTER,
     AV_CH_BACK_LEFT,
     AV_CH_BACK_RIGHT,
-    AV_CH_BACK_LEFT,           /* side surround left -- dup sur rear L */
-    AV_CH_BACK_RIGHT,          /* side surround right -- dup sur rear R */
+    AV_CH_SIDE_LEFT,           /* side surround left -- dup sur side L */
+    AV_CH_SIDE_RIGHT,          /* side surround right -- dup sur side R */
     AV_CH_FRONT_LEFT_OF_CENTER,
     AV_CH_FRONT_RIGHT_OF_CENTER,
     AV_CH_TOP_FRONT_LEFT,