iio:st_magn: Remove redundant call to iio_sw_buffer_preenable().

The equivalent of iio_sw_buffer_preenable() is now done in the IIO buffer core,
so there is no need to do this from the driver anymore.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/magnetometer/st_magn_buffer.c

@@ -25,16 +25,7 @@
 
 static int st_magn_buffer_preenable(struct iio_dev *indio_dev)
 {
-	int err;
-
-	err = st_sensors_set_enable(indio_dev, true);
-	if (err < 0)
-		goto st_magn_set_enable_error;
-
-	err = iio_sw_buffer_preenable(indio_dev);
-
-st_magn_set_enable_error:
-	return err;
+	return st_sensors_set_enable(indio_dev, true);
 }
 
 static int st_magn_buffer_postenable(struct iio_dev *indio_dev)