ASoC: rsnd: fixup 6ch settings to 8ch

rsnd need to use 8ch clock settings for 6ch for TDM.
Otherwise, it can't work correctly.
This patch fixup it.

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