ASoC: fsl_spdif: Silence output on stop

[ Upstream commit 0e4c2b6b ]

Clear TX registers on stop to prevent the SPDIF interface from sending
last written word over and over again.

Fixes: a2388a49

 ("ASoC: fsl: Add S/PDIF CPU DAI driver")
Signed-off-by: Matus Gajdos <matuszpd@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20230719164729.19969-1-matuszpd@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>