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>

drivers/iio/dac/stm32-dac-core.c

@@ -147,8 +147,16 @@ static int stm32_dac_probe(struct platform_device *pdev)
 	priv->common.vref_mv = ret / 1000;
 	dev_dbg(dev, "vref+=%dmV\n", priv->common.vref_mv);
 
-	rst = devm_reset_control_get_exclusive(dev, NULL);
-	if (!IS_ERR(rst)) {
+	rst = devm_reset_control_get_optional_exclusive(dev, NULL);
+	if (rst) {
+		if (IS_ERR(rst)) {
+			ret = PTR_ERR(rst);
+			if (ret != -EPROBE_DEFER)
+				dev_err(dev, "reset get failed, %d\n", ret);
+
+			goto err_hw_stop;
+		}
+
 		reset_control_assert(rst);
 		udelay(2);
 		reset_control_deassert(rst);