iio: adc: stm32-dfsdm: manage the get_irq error case

During probe, check the "get_irq" error value.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Acked-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>