drm/dp-mst-helper: Avoid reading uninitialized value

A static analysis tool tells me that we could try to read an
uninitialized 'ret' value. Plug that.
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -1644,7 +1644,7 @@ int drm_dp_update_payload_part2(struct drm_dp_mst_topology_mgr *mgr)
 {
 	struct drm_dp_mst_port *port;
 	int i;
-	int ret;
+	int ret = 0;
 	mutex_lock(&mgr->payload_lock);
 	for (i = 0; i < mgr->max_payloads; i++) {