Fix the following sparse warnings:
  CHECK   drivers/iio/adc/stm32-dfsdm-adc.c
symbol 'stm32_dfsdm_get_buff_cb' was not declared. Should it be static?
symbol 'stm32_dfsdm_release_buff_cb' was not declared. Should it be static?

BTW, move interrupt.h to sort headers alphabetically.
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>