realtek: fix standalone ports in presence of static fdb entries

realtek: fix standalone ports in presence of static fdb entries

The registers L2_PORT_STATIC_MV_ACT seem to specify the action to take
when the source address of a packet exists as a static fdb entry on
another port. By default the configured action is to drop such packets.

For standalone ports, this behaviour is undesired, as all traffic should
be forwarded to the CPU. So change the action to forward on standalone
ports.

A situation where this issue can occur is when a non-offloaded bond
interface is part of a bridge. In that case, the CPU port will have fdb
entries for devices connected to the bond interface, which are managed
by the assisted learning feature.

For now, this is only implemented for RTL838x/RTL839x, as the available
set of registers differs for the other devices.

Signed-off-by: Jan Hoffmann <jan@3e8.eu>