ASoC: rsnd: rsnd_ssi_is_multi_slave() macro uses rsnd_ssi_multi_slaves()

b4c83b17 ("ASoC: rsnd: add Multi channel support") added
Multi channel support, and current rsnd_ssi_is_multi_slave()'s check
method is !SSI equals SSIM1/2/3. But, SSI parent also hit to this macro.
Because of this reason, some stream which needs SSI parent clock
can't work correctly. This patch uses rsnd_ssi_multi_slaves() to
solve this issue. This issue was reported by Dung.

Reported-by: Nguyen Viet Dung <nv-dung@jinso.co.jp>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>