hwmon: (adc128d818) Fix advanced configuration register init

If the operation mode is non-zero and an external reference voltage is set,
first the operation mode is written to the advanced configuration register,
followed by the externel reference enable bit,
resetting the configuration mode to 0.

To fix this, first compose the value of the advanced configuration register
based on the configuration mode and the external reference voltage.
The advanced configuration register is then written to the device,
if it is different from the default register value (0x0).
Signed-off-by: Roy van Doormaal <roy.van.doormaal@prodrive-technologies.com>
Link: https://lore.kernel.org/r/20200728151846.231785-1-roy.van.doormaal@prodrive-technologies.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/adc128d818.c

@@ -393,6 +393,7 @@ static int adc128_init_client(struct adc128_data *data)
 {
 	struct i2c_client *client = data->client;
 	int err;
+	u8 regval = 0x0;
 
 	/*
 	 * Reset chip to defaults.
@@ -403,10 +404,17 @@ static int adc128_init_client(struct adc128_data *data)
 		return err;
 
 	/* Set operation mode, if non-default */
-	if (data->mode != 0) {
-		err = i2c_smbus_write_byte_data(client,
-						ADC128_REG_CONFIG_ADV,
-						data->mode << 1);
+	if (data->mode != 0)
+		regval |= data->mode << 1;
+
+	/* If external vref is selected, configure the chip to use it */
+	if (data->regulator)
+		regval |= 0x01;
+
+	/* Write advanced configuration register */
+	if (regval != 0x0) {
+		err = i2c_smbus_write_byte_data(client, ADC128_REG_CONFIG_ADV,
+						regval);
 		if (err)
 			return err;
 	}
@@ -416,14 +424,6 @@ static int adc128_init_client(struct adc128_data *data)
 	if (err)
 		return err;
 
-	/* If external vref is selected, configure the chip to use it */
-	if (data->regulator) {
-		err = i2c_smbus_write_byte_data(client,
-						ADC128_REG_CONFIG_ADV, 0x01);
-		if (err)
-			return err;
-	}
-
 	return 0;
 }