drm/bridge: ptn3460: Fix sparse warnings

The ptn3460_bridge_attach symbol is never used outside this file, so it
should be static.

Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/drm/bridge/ptn3460.c

@@ -265,7 +265,7 @@ static struct drm_connector_funcs ptn3460_connector_funcs = {
 	.destroy = ptn3460_connector_destroy,
 };
 
-int ptn3460_bridge_attach(struct drm_bridge *bridge)
+static int ptn3460_bridge_attach(struct drm_bridge *bridge)
 {
 	struct ptn3460_bridge *ptn_bridge = bridge_to_ptn3460(bridge);
 	int ret;