net: airoha: Fix PPE cpu port configuration for GDM2 loopback path

net: airoha: Fix PPE cpu port configuration for GDM2 loopback path

When QoS loopback is enabled for GDM3 or GDM4, incoming packets are
forwarded to GDM2. However, the PPE cpu port for GDM2 is not configured
in this path, causing traffic originating from GDM3/GDM4, which may
be set up as WAN ports backed by QDMA1, to be incorrectly directed
to QDMA0 instead.
Configure the PPE cpu port for GDM2 when QoS loopback is active on
GDM3 or GDM4 to ensure traffic is routed to the correct QDMA instance.

Fixes: 9cd451d414f6 ("net: airoha: Add loopback support for GDM2")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20260417-airoha-ppe-cpu-port-for-gdm2-loopback-v1-1-c7a9de0f6f57@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>