This patch implements .port_pre_bridge_flags() and .port_bridge_flags(),
which are required for properly treating the BR_LEARNING flag. Also,
.port_stp_state_set() is tweaked and now disables learning for standalone
ports.
Disabling learning for standalone ports is required to avoid situations
where one port sees traffic originating from another, which could cause
invalid operations.
Signed-off-by: Pawel Dembicki <paweldembicki@xxxxxxxxx>