ASoC: rsnd: tidyup debug information when read/write

b8c637864a6904a9ba8e0df556d5bdf9f26b2c54
(ASoC: rsnd: use regmap_mmio instead of original regmap bus)
added regmap_mmio support on Renesas R-Car sound driver.
Then, debug information of register read/write
indicates regmap index, not register address.
This is a little bit confusable information.
This patch tidyup debug message, and added regmap debug hint
on comment area.

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