net: bridge: add helper to set topology change

Add a __br_set_topology_change helper to set the topology change value.

This can be later extended to add actions when the topology change flag
is set or cleared.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>