regulator: Set apply_uV only when min and max voltages are defined

apply_uV is errornously set when regulator is instantiated from device
tree, even when it doesn't contain any voltage constraints.

This commit fixes error:

  machine_constraints_voltage: CHARGER: failed to apply 0uV constraint

for following regulator description in DTS:

  CHARGER {
	regulator-min-microamp = <100000>;
	regulator-max-microamp = <200000>;
  }
Signed-off-by: Karol Lewandowski <k.lewandowsk@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

drivers/regulator/of_regulator.c

@@ -35,7 +35,7 @@ static void of_get_regulation_constraints(struct device_node *np,
 	if (constraints->min_uV != constraints->max_uV)
 		constraints->valid_ops_mask |= REGULATOR_CHANGE_VOLTAGE;
 	/* Only one voltage?  Then make sure it's set. */
-	if (constraints->min_uV == constraints->max_uV)
+	if (min_uV && max_uV && constraints->min_uV == constraints->max_uV)
 		constraints->apply_uV = true;
 
 	uV_offset = of_get_property(np, "regulator-microvolt-offset", NULL);