drm/nouveau: use NULL for pointer assignment.

Replace the use of 0 in the pointer assignment with NULL to address the
following sparse warning:
drivers/gpu/drm/nouveau/nouveau_hwmon.c:744:29: warning: Using plain integer as NULL pointer

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_hwmon.c

@@ -741,7 +741,7 @@ nouveau_hwmon_init(struct drm_device *dev)
 			special_groups[i++] = &pwm_fan_sensor_group;
 	}
 
-	special_groups[i] = 0;
+	special_groups[i] = NULL;
 	hwmon_dev = hwmon_device_register_with_info(dev->dev, "nouveau", dev,
 							&nouveau_chip_info,
 							special_groups);