ASoC: rsnd: ssi enables non-stereo sound

Current SSI is assuming that the sound is always stereo. But, SSI needs
to calculate its frequency when master mode. Then This frequency depends
on each SSI's slots, and TDM mode (= TDM Extend Mode, TDM split Mode,
TDM Multichannel Mode).
This patch enables to use non-stereo sound.

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