net: dsa: rzn1-a5psw: use a5psw_reg_rmw() to modify flooding resolution

.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@bootlin.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>