hwmon: (adt7462) ADT7462_REG_VOLT_MAX() should return 0

The callers never check for negative but they sometimes check for zero
returns:

	int reg = ADT7462_REG_VOLT_MAX(data, i);
        data->volt_max[i] =
		(reg ? i2c_smbus_read_byte_data(client, reg) : 0);

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>