ASoC: Enable rbtree compression for WM8994/58 register cache

The WM8994 and WM8958 register map is relatively sparse so benefits
from compression. The rbtree compression gives better results than
LZO for both memory and CPU consumption on a map as sparse as this.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>

sound/soc/codecs/wm8994.c

@@ -3033,6 +3033,7 @@ static struct snd_soc_codec_driver soc_codec_dev_wm8994 = {
 	.reg_cache_size = WM8994_CACHE_SIZE,
 	.reg_cache_default = wm8994_reg_defaults,
 	.reg_word_size = 2,
+	.compress_type = SND_SOC_RBTREE_COMPRESSION,
 };
 
 static int __devinit wm8994_probe(struct platform_device *pdev)