iio: adc: stm32-dfsdm: fix single conversion

Apply data formatting to single conversion,
as this is already done in continuous and trigger modes.

Fixes: 102afde6 ("iio: adc: stm32-dfsdm: manage data resolution in trigger mode")

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Cc: <Stable@vger.kernel.org>
Acked-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>