pcm/Dsd32: fix the byte order

The byte order of DSD_U32 was wrong from the start.  The oldest bits
must be in the MSB, not in the LSB, according to
snd_pcm_format_descriptions in alsa-lib.

NEWS

@@ -5,6 +5,7 @@ ver 0.20.2 (not yet released)
   - libsamplerate: reset state after seeking
 * output
   - alsa: fix the DSD_U32 sample rate
+  - alsa: fix the DSD_U32 byte order
 
 ver 0.20.1 (2017/01/09)
 * input

src/pcm/Dsd32.cxx

@@ -29,8 +29,11 @@
 static constexpr inline uint32_t
 Construct32(uint8_t a, uint8_t b, uint8_t c, uint8_t d)
 {
-	return uint32_t(a) | (uint32_t(b) << 8) |
-		(uint32_t(c) << 16) | (uint32_t(d) << 24);
+	/* "a" is the oldest byte, which must be in the most
+	   significant byte */
+
+	return uint32_t(d) | (uint32_t(c) << 8) |
+		(uint32_t(b) << 16) | (uint32_t(a) << 24);
 }
 
 static constexpr inline uint32_t

src/pcm/Dsd32.hxx

@@ -28,7 +28,7 @@ template<typename T> struct ConstBuffer;
 class PcmBuffer;
 
 /**
- * Convert DSD_U8 to DSD_U32 (native endian).
+ * Convert DSD_U8 to DSD_U32 (native endian, oldest bits in MSB).
  */
 ConstBuffer<uint32_t>
 Dsd8To32(PcmBuffer &buffer, unsigned channels, ConstBuffer<uint8_t> src);