ASoC: wm8974: set cache type for regmap

Attempting to use this codec driver triggers a BUG() in regcache_sync()
since no cache type is set.  The register map of this device is fairly
small and has few holes so a flat cache is suitable.
Signed-off-by: Mans Rullgard <mans@mansr.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org

sound/soc/codecs/wm8974.c

@@ -574,6 +574,7 @@ static const struct regmap_config wm8974_regmap = {
 	.max_register = WM8974_MONOMIX,
 	.reg_defaults = wm8974_reg_defaults,
 	.num_reg_defaults = ARRAY_SIZE(wm8974_reg_defaults),
+	.cache_type = REGCACHE_FLAT,
 };
 
 static int wm8974_probe(struct snd_soc_codec *codec)