ASoC: rsnd: add rsnd_ssi_config_init()

In order to enhance code readability, this patch adds
rsnd_ssi_config_init() and moves SSICR register settings to it.
This is prepare patch for TDM support

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