ASoC: rsnd: add rsnd_daidrv_get()

rsnd priv has many parameters. On __rsnd_dai_probe() it uses
rsnd_rdai_get() to get rdai pointer, but is using priv->daidrv
directly to get daidrvhv, but it is confusable for reader.
This patch adds rsnd_daidrv_get() to get daidrv from priv.
Now reader can understand that rdai and daidrv are related.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>