net: dsa: rzn1-a5psw: enable management frames for CPU port

Currently, management frame were discarded before reaching the CPU port due
to a misconfiguration of the MGMT_CONFIG register. Enable them by setting
the correct value in this register in order to correctly receive management
frame and handle STP.

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>