hwmon: (adt7475) Fix build warning

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

drivers/hwmon/adt7475.c:171:34: warning:
	‘adt7475_of_match’ defined but not used

Mark adt7475_of_match as __maybe_unused to fix the problem.

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