iio: adc: stm32-dfsdm: missing error case during probe

During probe, check the devm_ioremap_resource() error value.
Also return the devm_clk_get() error value instead of -EINVAL.

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>