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.com


Signed-off-by: Mark Brown <broonie@kernel.org>