bridge: check bridge port vlan membership on link-up events

bridge: check bridge port vlan membership on link-up events

When changing to a dfs channel, hostapd can bring down wlan interfaces and
reset their bridge membership. If that happens, the port loses its vlan
membership settings and needs to be reconfigured by netifd.

Signed-off-by: Felix Fietkau <nbd@nbd.name>