hwmon: (adc128d818) Preserve operation mode

Preserve chip operation mode if no mode is specified via devicetree. This
enables operation when chip configuration is done by BIOS/ROMMON.

Signed-off-by: Alexander Koch <mail@alexanderkoch.net>
Acked-by: Michael Hornung <mhornung.linux@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>