hwmon: (stts751) Fix build warning

The following build warning is seen if CONFIG_OF is disabled.

drivers/hwmon/stts751.c:88:34: warning:
	‘stts751_of_match’ defined but not used

Mark stts751_of_match as __maybe_unused to fix the problem.

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