mfd: Put WM8994 into cache only mode when suspending

This is required by the ASoC driver for very low power modes where the
device is fully idle but we want to update controls.
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>

drivers/mfd/wm8994-core.c

@@ -268,6 +268,7 @@ static int wm8994_suspend(struct device *dev)
 	wm8994_reg_write(wm8994, WM8994_SOFTWARE_RESET,
 			 wm8994_reg_read(wm8994, WM8994_SOFTWARE_RESET));
 
+	regcache_cache_only(wm8994->regmap, true);
 	regcache_mark_dirty(wm8994->regmap);
 
 	wm8994->suspended = true;
@@ -298,6 +299,7 @@ static int wm8994_resume(struct device *dev)
 		return ret;
 	}
 
+	regcache_cache_only(wm8994->regmap, false);
 	ret = regcache_sync(wm8994->regmap);
 	if (ret != 0) {
 		dev_err(dev, "Failed to restore register map: %d\n", ret);