ASoC: simple-scu-card: tidyup "convert-rate/channels" parsing

simple-scu-card.c is supporting "convert-rate/channels" which is
used for DPCM.
But, sound card might have multi codecs, and each codec might need
each convert-rate/channels.

This patch supports each codec's convert-rate/channles support.
top node convert-rate/channels will overwrite settings if exist.

It can't support each codec's convert-rate/channels if sound card had
multi codecs without this patch.

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