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>