ASoC: soc_codec_reg_show use snd_soc_codec_readable_register

Use snd_soc_codec_readable_register instead of open-coding it.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>