ASoC: wm8988: Use snd_soc_update_bits for read-modify-write

Use snd_soc_update_bits for read-modify-write register access instead of
open-coding it using snd_soc_read and snd_soc_write
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

sound/soc/codecs/wm8988.c

@@ -759,7 +759,6 @@ static int wm8988_probe(struct snd_soc_codec *codec)
 	struct wm8988_priv *wm8988 = snd_soc_codec_get_drvdata(codec);
 	struct snd_soc_dapm_context *dapm = &codec->dapm;
 	int ret = 0;
-	u16 reg;
 
 	ret = snd_soc_codec_set_cache_io(codec, 7, 9, wm8988->control_type);
 	if (ret < 0) {