realtek: dsa: rtl93xx: Implement vlan fast age flushing

realtek: dsa: rtl93xx: Implement vlan fast age flushing

The DSA port code is trying to flush associated VLANs whenever the MST
state is changed. This functionality is available on a per port+vid based
using the L2_TBL_FLUSH_CTRL which is already used for the .port_fast_age
callbacks.

Signed-off-by: Issam Hamdi <ih@simonwunderlich.de>
Co-developed-by: Sven Eckelmann <se@simonwunderlich.de>
Signed-off-by: Sven Eckelmann <se@simonwunderlich.de>
Link: https://github.com/openwrt/openwrt/pull/20421
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>