hwmon: (max6621) Fix build warning

The following build warning is seen if CONFIG_OF is disabled.

drivers/hwmon/max6621.c:573:34: warning:
	‘max6621_of_match’ defined but not used

Mark max6621_of_match as __maybe_unused to fix the problem.

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