iio: dac: ad5592r: localize locks only where needed in ad5592r_read_raw()

Since there was a recently discovered issue with these locks, it probably
makes sense to cleanup the code a bit, to prevent it from being used as an
example/reference.

This change moves the lock only where it is explicitly needed to protect
resources from potential concurrent accesses.

It also reworks the switch statements to do direct returns vs caching the
return value on a variable.
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20200706110259.23947-3-alexandru.ardelean@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/dac/ad5592r-base.c

@@ -380,32 +380,32 @@ static int ad5592r_read_raw(struct iio_dev *iio_dev,
 
 	switch (m) {
 	case IIO_CHAN_INFO_RAW:
-		mutex_lock(&st->lock);
-
 		if (!chan->output) {
+			mutex_lock(&st->lock);
 			ret = st->ops->read_adc(st, chan->channel, &read_val);
+			mutex_unlock(&st->lock);
 			if (ret)
-				goto unlock;
+				return ret;
 
 			if ((read_val >> 12 & 0x7) != (chan->channel & 0x7)) {
 				dev_err(st->dev, "Error while reading channel %u\n",
 						chan->channel);
-				ret = -EIO;
-				goto unlock;
+				return -EIO;
 			}
 
 			read_val &= GENMASK(11, 0);
 
 		} else {
+			mutex_lock(&st->lock);
 			read_val = st->cached_dac[chan->channel];
+			mutex_unlock(&st->lock);
 		}
 
 		dev_dbg(st->dev, "Channel %u read: 0x%04hX\n",
 				chan->channel, read_val);
 
 		*val = (int) read_val;
-		ret = IIO_VAL_INT;
-		break;
+		return IIO_VAL_INT;
 	case IIO_CHAN_INFO_SCALE:
 		*val = ad5592r_get_vref(st);
 
@@ -425,11 +425,13 @@ static int ad5592r_read_raw(struct iio_dev *iio_dev,
 			mult = !!(st->cached_gp_ctrl &
 				AD5592R_REG_CTRL_ADC_RANGE);
 
+		mutex_unlock(&st->lock);
+
 		*val *= ++mult;
 
 		*val2 = chan->scan_type.realbits;
-		ret = IIO_VAL_FRACTIONAL_LOG2;
-		break;
+
+		return IIO_VAL_FRACTIONAL_LOG2;
 	case IIO_CHAN_INFO_OFFSET:
 		ret = ad5592r_get_vref(st);
 
@@ -439,15 +441,13 @@ static int ad5592r_read_raw(struct iio_dev *iio_dev,
 			*val = (-34365 * 25) / ret;
 		else
 			*val = (-75365 * 25) / ret;
-		ret =  IIO_VAL_INT;
-		break;
+
+		mutex_unlock(&st->lock);
+
+		return IIO_VAL_INT;
 	default:
 		return -EINVAL;
 	}
-
-unlock:
-	mutex_unlock(&st->lock);
-	return ret;
 }
 
 static int ad5592r_write_raw_get_fmt(struct iio_dev *indio_dev,