hwmon: (ad7414) Fix build warning

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

drivers/hwmon/ad7414.c:218:34: warning:
	‘ad7414_of_match’ defined but not used

Marking ad7414_of_match as __mayybe_unused fixes the problem.

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