IIO: ADC: stm32-dfsdm: code optimization

Use of_device_get_match_data to optimize the source code.
No check is needed on dev_data as match table is defined in driver.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>