feat(st-regulator): support regulator_set_voltage for fixed regulator

Always support the regulator_set_voltage operation for the same voltage.
This patch prepares the DDR power configuration when the power supplies
are fixed.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Maxime Méré <maxime.mere@foss.st.com>
Change-Id: Idf032b03c6f0c95f997dec3ed8a0d38c54a3de15
diff --git a/drivers/st/regulator/regulator_core.c b/drivers/st/regulator/regulator_core.c
index 0b11e53..b369acd 100644
--- a/drivers/st/regulator/regulator_core.c
+++ b/drivers/st/regulator/regulator_core.c
@@ -215,14 +215,18 @@
 
 	VERBOSE("%s: set mvolt\n", rdev->desc->node_name);
 
-	if (rdev->desc->ops->set_voltage == NULL) {
-		return -ENODEV;
-	}
-
 	if ((mvolt < rdev->min_mv) || (mvolt > rdev->max_mv)) {
 		return -EPERM;
 	}
 
+	if (regulator_get_voltage(rdev) == mvolt) {
+		return 0U;
+	}
+
+	if (rdev->desc->ops->set_voltage == NULL) {
+		return -ENODEV;
+	}
+
 	lock_driver(rdev);
 
 	ret = rdev->desc->ops->set_voltage(rdev->desc, mvolt);