IIO: ADC: stm32_dfsdm_stop_filter() can be static

Fixes: e2e6771c ("IIO: ADC: add STM32 DFSDM sigma delta ADC support")
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>