ASoC: Implement new DC servo readback mode for late WM8994 revisions

Later WM8994 devices implement a new DC servo readback mode with the
register used to access the offset moved to register 0x59. Implement
support for this and enable it on the appropriate devices.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>