Remove WM8731 TODO comment (#623)

I was reading the WM8731 audio codec setup code and I got confused by
a comment which suggested Daisy always uses the codec with a 48kHz
sample rate. After several hours of reading data sheets I figured out
what is going on here: the WM8731 has built-in sample rate conversion
and the nominal "48kHz" setting disables the built-in conversion.

This commit updates the comment to explain what we are actually doing here.

Author: jacobvosmaer

src/dev/codec_wm8731.cpp

@@ -77,12 +77,6 @@ enum CodecSettings
     CODEC_FORMAT_MASTER       = 0x40,
     CODEC_FORMAT_SLAVE        = 0x00,
     CODEC_FORMAT_INVERT_CLOCK = 0x80,
-
-    CODEC_RATE_48K_48K = 0x00 << 2,
-    CODEC_RATE_8K_8K   = 0x03 << 2,
-    CODEC_RATE_96K_96K = 0x07 << 2,
-    CODEC_RATE_32K_32K = 0x06 << 2,
-    CODEC_RATE_44K_44K = 0x08 << 2,
 };
 
 Wm8731::Result Wm8731::Init(const Wm8731::Config &config, I2CHandle i2c)
@@ -149,9 +143,14 @@ Wm8731::Result Wm8731::Init(const Wm8731::Config &config, I2CHandle i2c)
     if(res != Result::OK)
         return Result::ERR;
 
-    // samplerate
-    // TODO: add support for other samplerates
-    res = WriteControlRegister(CODEC_REG_SAMPLE_RATE, CODEC_RATE_48K_48K);
+    // The WM8731 is designed to run from a fixed rate master clock and it
+    // has its own clock rate conversion mechanisms to derive various sample
+    // rates from the fixed master clock. In Daisy however we use a variable
+    // rate master clock generated by the STM32 SAI peripheral so we do not
+    // want this extra rate conversion. By setting the WM8731 sample rate
+    // register to 0, we disable the WM8731's own internal sample rate
+    // conversion.
+    res = WriteControlRegister(CODEC_REG_SAMPLE_RATE, 0x00);
     if(res != Result::OK)
         return Result::ERR;