realtek: dsa: Automatically return lost VLANs to CIST

If a VLAN doesn't have any members anymore, then it is removed and
implicitly returns back from any MSTI to CIST. The DSA layer will not
create any call to .vlan_msti_set and the driver is required to handle this
directly.

Suggested-by: Jonas Gorski <[email protected]>
Signed-off-by: Sven Eckelmann <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20421
Signed-off-by: Hauke Mehrtens <[email protected]>

target/linux/realtek/files-6.12/drivers/net/dsa/rtl83xx/dsa.c

@@ -1819,6 +1819,10 @@ static int rtl83xx_vlan_del(struct dsa_switch *ds, int port,
 	info.untagged_ports &= (~BIT_ULL(port));
 	info.member_ports &= (~BIT_ULL(port));
 
+	/* VLANs without members are set back (implicitly) to CIST by DSA */
+	if (!info.member_ports)
+		info.fid = 0;
+
 	priv->r->vlan_set_untagged(vlan->vid, info.untagged_ports);
 	pr_debug("Untagged ports, VLAN %d: %llx\n", vlan->vid, info.untagged_ports);