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/dd4bf6aSigned-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org