realtek: dsa: Share port member configuration code

realtek: dsa: Share port member configuration code

The leave and join callbacks for DSA were using their own implementation of
the port member handling code. This makes the implementation of additional
functionality based on the port member matrix complicated because it needs
to be implemented in both places and also in the new code path for the
introduced feature.

By sharing this code, it is much easier to guarantee that all code paths
behave the same. This approach is already implemented by other DSA drivers
like qca8k, mt7530 or ksz.

Signed-off-by: Sven Eckelmann <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20360
Signed-off-by: Robert Marko <[email protected]>