If the zero duty cycle doesn't correspond to any voltage in the voltage
table, the PWM regulator returns an -EINVAL from get_voltage_sel() which
results in the core erroring out with a "failed to get the current
voltage" and ending up not applying the machine constraints.

Instead, return -ENOTRECOVERABLE which makes the core set the voltage
since it's at an unknown value.

For example, with this device tree:

	fooregulator {
		compatible = "pwm-regulator";
		pwms = <&foopwm 0 100000>;
		regulator-min-microvolt = <2250000>;
		regulator-max-microvolt = <2250000>;
		regulator-name = "fooregulator";
		regulator-always-on;
		regulator-boot-on;
		voltage-table = <2250000 30>;
	};

Before this patch:

  fooregulator: failed to get the current voltage(-22)

After this patch:

  fooregulator: Setting 2250000-2250000uV
  fooregulator: 2250 mV
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Link: https://lore.kernel.org/r/20200902130952.24880-1-vincent.whitchurch@axis.comSigned-off-by: Mark Brown <broonie@kernel.org>