ASoC: rsnd: use regmap_mmio instead of original regmap bus

Current rsnd driver is using regmap and regmap_field.
It used original regmap bus which is
single regmap instance for multi register mapping.
This patch modifies it to use regmap_mmio bus,
and tidyuped probe method
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

sound/soc/sh/Kconfig

@@ -37,7 +37,7 @@ config SND_SOC_SH4_SIU
 config SND_SOC_RCAR
 	tristate "R-Car series SRU/SCU/SSIU/SSI support"
 	select SND_SIMPLE_CARD
-	select REGMAP
+	select REGMAP_MMIO
 	help
 	  This option enables R-Car SUR/SCU/SSIU/SSI sound support