ASoC: Support second DC servo readback method for wm_hubs

More recent Wolfson hubs devices add the ability to read back the DC
servo calibration information from the register used to write offsets,
and later still ones remove the old readback registers. Add support
for the new scheme, and use it for WM8994 device revisions that
support it.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
diff --git a/sound/soc/codecs/wm_hubs.h b/sound/soc/codecs/wm_hubs.h
index 420104f..e51c166 100644
--- a/sound/soc/codecs/wm_hubs.h
+++ b/sound/soc/codecs/wm_hubs.h
@@ -21,6 +21,7 @@
 /* This *must* be the first element of the codec->private_data struct */
 struct wm_hubs_data {
 	int dcs_codes;
+	int dcs_readback_mode;
 	int hp_startup_mode;
 };