iio: dac: ad5446: Replace indio_dev->mlock with own device lock

As part of the general cleanup of indio_dev->mlock, this change replaces
it with a local lock on the device's state structure.
Signed-off-by: Sergiu Cuciurean <sergiu.cuciurean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/dac/ad5446.c

@@ -33,6 +33,7 @@
  * @chip_info:		chip model specific constants, available modes etc
  * @reg:		supply regulator
  * @vref_mv:		actual reference voltage used
+ * @lock		lock to protect the data buffer during write ops
  */
 
 struct ad5446_state {
@@ -43,6 +44,7 @@ struct ad5446_state {
 	unsigned			cached_val;
 	unsigned			pwr_down_mode;
 	unsigned			pwr_down;
+	struct mutex			lock;
 };
 
 /**
@@ -112,7 +114,7 @@ static ssize_t ad5446_write_dac_powerdown(struct iio_dev *indio_dev,
 	if (ret)
 		return ret;
 
-	mutex_lock(&indio_dev->mlock);
+	mutex_lock(&st->lock);
 	st->pwr_down = powerdown;
 
 	if (st->pwr_down) {
@@ -123,7 +125,7 @@ static ssize_t ad5446_write_dac_powerdown(struct iio_dev *indio_dev,
 	}
 
 	ret = st->chip_info->write(st, val);
-	mutex_unlock(&indio_dev->mlock);
+	mutex_unlock(&st->lock);
 
 	return ret ? ret : len;
 }
@@ -197,11 +199,11 @@ static int ad5446_write_raw(struct iio_dev *indio_dev,
 			return -EINVAL;
 
 		val <<= chan->scan_type.shift;
-		mutex_lock(&indio_dev->mlock);
+		mutex_lock(&st->lock);
 		st->cached_val = val;
 		if (!st->pwr_down)
 			ret = st->chip_info->write(st, val);
-		mutex_unlock(&indio_dev->mlock);
+		mutex_unlock(&st->lock);
 		break;
 	default:
 		ret = -EINVAL;
@@ -256,6 +258,8 @@ static int ad5446_probe(struct device *dev, const char *name,
 	indio_dev->channels = &st->chip_info->channel;
 	indio_dev->num_channels = 1;
 
+	mutex_init(&st->lock);
+
 	st->pwr_down_mode = MODE_PWRDWN_1k;
 
 	if (st->chip_info->int_vref_mv)