ASoC: fsl-asoc-card: Add comment for mclk in the codec_priv

Otherwise a warning will be detected as below:
warning: Function parameter or member 'mclk' not described in
'codec_priv'

Fixes: 1075df4b ("ASoC: fsl-asoc-card: add nau8822 support")
Signed-off-by: Hui Wang <hui.wang@canonical.com>
Link: https://lore.kernel.org/r/20231007040117.22446-1-hui.wang@canonical.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/fsl/fsl-asoc-card.c

@@ -41,6 +41,7 @@
 
 /**
  * struct codec_priv - CODEC private data
+ * @mclk: Main clock of the CODEC
  * @mclk_freq: Clock rate of MCLK
  * @free_freq: Clock rate of MCLK for hw_free()
  * @mclk_id: MCLK (or main clock) id for set_sysclk()