net: ethernet: mtk_eth_soc: add code for offloading flows from wlan devices

WED version 2 (on MT7986 and later) can offload flows originating from
wireless devices.
In order to make that work, ndo_setup_tc needs to be implemented on the
netdevs. This adds the required code to offload flows coming in from WED,
while keeping track of the incoming wed index used for selecting the
correct PPE device.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>