hwmon: (hwmon-vid) Add __maybe_unused attribute to dummy variable

This gets rid of this warning:

drivers/hwmon/hwmon-vid.c: In function 'get_via_model_d_vrm':
drivers/hwmon/hwmon-vid.c:249:27: warning: variable 'dummy' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Jean Delvare <khali@linux-fr.org>