net: dsa: rzn1-a5psw: disable learning for standalone ports

When ports are in standalone mode, they should have learning disabled to
avoid adding new entries in the MAC lookup table which might be used by
other bridge ports to forward packets. While adding that, also make sure
learning is enabled for CPU port.

Fixes: 888cdb89

 ("net: dsa: rzn1-a5psw: add Renesas RZ/N1 advanced 5 port switch driver")
Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Reviewed-by: Piotr Raczynski <piotr.raczynski@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>