hwmon: (adc128d818) Fix build warning

If CONFIG_OF is not enabled, the following build warning is observed.

drivers/hwmon/adc128d818.c:524:34: warning:
	‘adc128_of_match’ defined but not used

Mark adc128_of_match as __maybe_unused to fix the problem.

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