ASoC: fsl_spdif: STL and STR registers are non volatile

STL and STR registers are writable and not readable. So they are non volatile.
Remove them from volatile list, and add default register value for them.

Signed-off-by: Zidan Wang <zidan.wang@freescale.com>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>