regulator: core: Fix the init of DT defined fixed regulators

When a regulator is defined using DT and it has a single voltage the
regulator init always tries to apply this voltage. However it fails if
the regulator isn't settable because it is using an internal low level
function. To overcome this we now first query the regulator and only
set it if needed.
Signed-off-by: Alban Bedel <alban.bedel@avionic-design.de>
Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/regulator/core.c

@@ -844,15 +844,24 @@ static int machine_constraints_voltage(struct regulator_dev *rdev,
 	/* do we need to apply the constraint voltage */
 	if (rdev->constraints->apply_uV &&
 	    rdev->constraints->min_uV == rdev->constraints->max_uV) {
-		ret = _regulator_do_set_voltage(rdev,
-						rdev->constraints->min_uV,
+		int current_uV = _regulator_get_voltage(rdev);
+		if (current_uV < 0) {
+			rdev_err(rdev, "failed to get the current voltage\n");
+			return current_uV;
+		}
+		if (current_uV < rdev->constraints->min_uV ||
+		    current_uV > rdev->constraints->max_uV) {
+			ret = _regulator_do_set_voltage(
+				rdev, rdev->constraints->min_uV,
 				rdev->constraints->max_uV);
 			if (ret < 0) {
-			rdev_err(rdev, "failed to apply %duV constraint\n",
+				rdev_err(rdev,
+					"failed to apply %duV constraint\n",
 					rdev->constraints->min_uV);
 				return ret;
 			}
 		}
+	}
 
 	/* constrain machine-level voltage specs to fit
 	 * the actual range supported by this regulator.