ASoC: rsnd: judge multi 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.
Current judgement is vague, and had broken by
c308abe4("ASoC: rsnd: rsnd_ssi_is_multi_slave() macro uses
rsnd_ssi_multi_slaves()")
This patch makes clean it, and solve this issue.

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