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>
(cherry picked from commit 2a0035113a0f16305476083fc6bacdd0e29b1e5f)