ASoC: soc-core: adjust for graph on snd_soc_of_parse_audio_prefix

It is assuming that the card related information is located on
"card" node, but graph case doesn't have it.
This patch adds node parameter to adjust for graph support

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