ASoC: fsl_spdif: Disable TX clock when stop

The TX clock source may be changed in next case, need to
disable it when stop, otherwise the TX may not work after
changing the clock source, error log is:

aplay: pcm_write:2058: write error: Input/output error

Fixes: a2388a49

 ("ASoC: fsl: Add S/PDIF CPU DAI driver")
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/1646879863-27711-1-git-send-email-shengjiu.wang@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>