ASoC: audio-graph-card: add asoc_graph_card_get_conversion()

audio-graph-card is now supporting normal sound and DPCM sound.
For DPCM sound, original sound card (= audio-graph-scu) had been
supported 1 CPU : 1 Codec connection which uses hw_params_fixup()
for convert-rate/channel.
But, merged audio-graph-card is completely forgeting about it.

To re-support 1 CPU : 1 Codec DPCM for hw_params_fixup(),
it need to judge whether it is DPCM by checking convert-rate/channel.
For this purpose, this patch adds asoc_graph_card_get_conversion()
as preparation

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