hwmon: (lm90) Fix build warning

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

drivers/hwmon/lm90.c:239:34: warning:
	‘lm90_of_match’ defined but not used

Mark lm90_of_match as __maybe_unused to fix the problem.

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