drm: drm_stub: Fixing return value if driver master_set call failed

When dev->driver->master_set() failed ioctl call return 0
but the caller is not the DRM-Master because file_priv->is_master = 0.
Fix that by returning to ioctl caller the driver master_set error code.

Signed-off-by: Benjamin Gaignard <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>

drivers/gpu/drm/drm_stub.c

···
 int drm_setmaster_ioctl(struct drm_device *dev, void *data,
 			struct drm_file *file_priv)
 {
-	int ret;
+	int ret = 0;
 
 	if (file_priv->is_master)
 		return 0;
···
 	}
 	mutex_unlock(&dev->struct_mutex);
 
-	return 0;
+	return ret;
 }
 
 int drm_dropmaster_ioctl(struct drm_device *dev, void *data,