hwmon: (ads1015) Fix build warning

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

drivers/hwmon/ads1015.c:310:34: warning:
	‘ads1015_of_match’ defined but not used

Marking ads1015_of_match as __maybe_unused fixes the problem.

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