iio: adc: stm32-dfsdm: fix compatible data use

Fix use of compatible data: stm32h7 regmap configuration is statically
used. Rather use regmap_cfg from compatible data.

Fixes: bed73904 ("IIO: ADC: add stm32 DFSDM core support")

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>