drm/nouveau/hwmon: fix compilation without CONFIG_HWMON

Reported-by: Jim Davis <jim.epost@gmail.com>
Tested-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Dave Airlie <airlied@redhat.com>