ASoC: fsl_spdif: Add kctl for configuring TX validity bit

Add one kctl for configuring TX validity bit from user
space.

The type of this kctl is boolean:
on - Outgoing validity always set
off - Outgoing validity always clear

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1594112066-31297-3-git-send-email-shengjiu.wang@nxp.com


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