ASoC: rockchip: spdif: Mark SPDIF_SMPDR as volatile

This patch marks SPDIF_SMPDR as volatile to make it resaonable,
which also requires marking it as readable, even though it isn't.

Signed-off-by: Sugar Zhang <sugar.zhang@rock-chips.com>
Link: https://lore.kernel.org/r/1629800162-12824-2-git-send-email-sugar.zhang@rock-chips.com


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