ASoC: wm8731: init mutex in i2c init path

The I2C init path forgot to init the mutex, leading to an oops when
controls are accessed.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org

sound/soc/codecs/wm8731.c

@@ -717,6 +717,8 @@ static int wm8731_i2c_probe(struct i2c_client *i2c,
 	if (wm8731 == NULL)
 		return -ENOMEM;
 
+	mutex_init(&wm8731->lock);
+
 	wm8731->regmap = devm_regmap_init_i2c(i2c, &wm8731_regmap);
 	if (IS_ERR(wm8731->regmap)) {
 		ret = PTR_ERR(wm8731->regmap);