mfd: Keep a cache of WM8350 volatile values

Due to the way that the WM8350 audio driver handles CODEC_ENA many of
the WM8350 audio registers are marked as volatile when they aren't
actually so. Allow the audio driver to see a cache of these values for
inspection during interrupt context.

To do this we need to stop satisfying any bits from volatile registers
from cache - there's no real benefit from doing so anyway, we did the
read already.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>