ASoC: rsnd: add rsnd_runtime_channel_xxx()

Current SSI is supporting Normal SSI/Multi mode SSI/TDM mode SSI
and its behavior is based on input channels.
This input channel might be converted by CTU,
and SSI needs to be Multi SSI mode / TDM SSI mode if 6ch input

EX) 6ch input, CTU for 2ch, playback

 6ch    6ch    2ch    2ch    2ch     2ch
 -> SRC -> CTU -> MIX -> DVC -> SSIU -> SSI

EX) 6ch input, no CTU, Multi SSI, playback

 6ch    6ch    6ch    6ch    6ch     2ch
 -> SRC -> CTU -> MIX -> DVC -> SSIU -> SSI0/SSI1/SSI2

Current driver is using rsnd_get_adinr_chan() / rsnd_get_slot_width()
for this purpose, but it is complicated enough without meaning.
This patch adds new rsnd_runtime_channel_xxx() which is caring
CTU/Multi SSI.

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