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.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>