iio: dac: stm32-dac: better handle reset controller failures

Use devm_reset_control_get_optional_exclusive() instead of
devm_reset_control_get_exclusive() as reset controller is optional.

Nevertheless if reset controller is expected but reports an
error, propagate the error code to the caller. In such case
a nice error trace is emitted unless we're deferring the probe
operation.

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