iio: dac: stm32-dac: use reset controller only at probe time

This change removes the reset controller reference from the local
DAC instance since it is used only at probe time.

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>