iio: adc: stm32-dfsdm: fix call to stop channel

stm32_dfsdm_stop_channel must be called with channel id, not filter id.

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