iio: adc: stm32-dfsdm: fix unmet direct dependencies detected

This fixes unmet direct dependencies seen when CONFIG_STM32_DFSDM_ADC
is selected:

WARNING: unmet direct dependencies detected for IIO_BUFFER_HW_CONSUMER
  Depends on [n]: IIO [=y] && IIO_BUFFER [=n]
  Selected by [y]:
  - STM32_DFSDM_ADC [=y] && IIO [=y] && (ARCH_STM32 [=y] && OF [=y] ||
    COMPILE_TEST [=n])

Fixes: e2e6771c ("IIO: ADC: add STM32 DFSDM sigma delta ADC support")

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