ASoC: cs4720: use snd_soc_cache_sync()

Replace the manual register restore mechanism in cs4270.c and call the
generic snd_soc_cache_sync() handler instead.

This factors code out in favour of core facilities and also fixes a
bus confusion that is most probably caused by intermixing i2c-regmap
functions and i2c_smbus_* accessors.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Reported-and-tested-by: Sven Neumann <s.neumann@raumfeld.com>
Acked-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>