ASoC: rsnd: judge work SSI in runtime

Current rsnd supports multi SSI (maximum 4 SSI for 8ch),
and, it should determine whether using each SSI or not in runtime.
All SSIs are not used even if there are 4 SSI in case of stereo.

Current driver setups un-used SSI in such case. It is no problem,
but not needed. This patch judges it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>