ASoC: wm8996: Inline wm8996_reset() and optimise cache-only usage

There is only one caller and this allows us to cleanly leave the CODEC
with the internal LDO powered down which is the default state we're
looking for and means that we can robustly disable the register cache
only when we either disable the LDO or power down the external
regulators.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>