ASoC: ad193x: Use snd_soc_update_bits where appropriate

We can reduce the code size here a bit by using snd_soc_update_bits instead of
open-coding the read-modify-write cycle. The conversion done in this patch is
not completely straightforward and some minor code restructuring has been
incorporated to further reduce the code size.

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