ASoC: rt722-sdca-sdw: add silence detection register as volatile

Including silence detection register as volatile.

Signed-off-by: Jack Yu <jack.yu@realtek.com>
Link: https://msgid.link/r/c66a6bd6d220426793096b42baf85437@realtek.com
Signed-off-by: Mark Brown <broonie@kernel.org>