.port_bridge_flags will be added and allows to modify the flood mask
independently for each port. Keeping the existing bridged_ports write
in a5psw_flooding_set_resolution() would potentially messed up this.
Use a read-modify-write to set that value and move bridged_ports
handling in bridge_port_join/leave.
Signed-off-by: Clément Léger <clement.leger@xxxxxxxxxxx>