iio: light: gp2ap020a00f: use iio helper function to lock direct mode

Replace the code that guarantees the device stays in direct mode
with iio_device_claim_direct_mode() which does same.  Includes a
tiny bit of refactoring (single case -> if) and simplified return
flow.

Also prevents a theoretical race condition by effectively taking the lock
a little earlier than before.

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Cc: Daniel Baluta <daniel.baluta@gmail.com>
Acked-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/light/gp2ap020a00f.c

@@ -1287,22 +1287,14 @@ static int gp2ap020a00f_read_raw(struct iio_dev *indio_dev,
 	struct gp2ap020a00f_data *data = iio_priv(indio_dev);
 	int err = -EINVAL;
 
-	mutex_lock(&data->lock);
-
-	switch (mask) {
-	case IIO_CHAN_INFO_RAW:
-		if (iio_buffer_enabled(indio_dev)) {
-			err = -EBUSY;
-			goto error_unlock;
-		}
+	if (mask == IIO_CHAN_INFO_RAW) {
+		err = iio_device_claim_direct_mode(indio_dev);
+		if (err)
+			return err;
 
 		err = gp2ap020a00f_read_channel(data, chan, val);
-		break;
+		iio_device_release_direct_mode(indio_dev);
 	}
-
-error_unlock:
-	mutex_unlock(&data->lock);
-
 	return err < 0 ? err : IIO_VAL_INT;
 }