iio: adc: stm32-dfsdm: claim direct mode for raw read and settings

Claim direct mode to ensure no buffer mode is in use for:
- single conversion
- sample rate setting (must be set when filter isn't enabled).
- oversampling ratio (must be set when filter isn't enabled).

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>