The SGTL5000_CHIP_ANA_POWER register is cached. Update the cached
value instead of writing it directly.
Patch inspired by Russell King's more colorful remarks in this
patch:
https://github.com/SolidRun/linux-imx6-3.14/commit/dd4bf6a
Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com> Signed-off-by: Mark Brown <broonie@kernel.org> Signed-off-by: Ben Hutchings <ben@decadent.org.uk>