net: ethernet: mtk_ppe_offload: Allow QinQ, double ETH_P_8021Q only

mtk_foe_entry_set_vlan() in mtk_ppe.c already supports double vlan
tagging, but mtk_flow_offload_replace() in mtk_ppe_offload.c only allows
for 1 vlan tag, optionally in combination with pppoe and dsa tags.

However, mtk_foe_entry_set_vlan() only allows for setting the vlan id.
The protocol cannot be set, it is always ETH_P_8021Q, for inner and outer
tag. This patch adds QinQ support to mtk_flow_offload_replace(), only in
the case that both inner and outer tags are ETH_P_8021Q.

Only PPPoE-in-Q (as before) and Q-in-Q are allowed. A combination
of PPPoE and Q-in-Q is not allowed.

Signed-off-by: Eric Woudstra <ericwouds@gmail.com>
Link: https://patch.msgid.link/20250225201509.20843-1-ericwouds@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>