regulator: pwm: Fix handling of missing init voltage

Since priv->init_voltage is an unsigned integer it can never be
negative.  So the current code fails to detect a missing
'regulator-init-microvolt' property and instead misconfigures the
PWM device.  Fix this by making the relevant members of
'struct pwm_regulator_info' signed integers.
Signed-off-by: NMark Kettenis <kettenis@openbsd.org>

drivers/power/regulator/pwm_regulator.c

@@ -32,13 +32,13 @@ struct pwm_regulator_info {
 	bool polarity;
 	struct udevice *pwm;
 	/* initialize voltage of regulator */
-	unsigned int init_voltage;
+	int init_voltage;
 	/* the maximum voltage of regulator */
-	unsigned int max_voltage;
+	int max_voltage;
 	/* the minimum voltage of regulator */
-	unsigned int min_voltage;
+	int min_voltage;
 	/* the current voltage of regulator */
-	unsigned int volt_uV;
+	int volt_uV;
 };
 
 static int pwm_regulator_enable(struct udevice *dev, bool enable)