iio: adc: stm32-dfsdm: add check on max filter id

reg property should be checked against number of available filters.
BTW, dfsdm->num_fls wasn't used. But it can be used for this purpose.
This prevents using data out of allocated dfsdm->fl_list array.

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