Optimized build for the D-Link DIR-860L

[Mushoz]

@r43k3n Wifi works fine for my usage, but I don't use heavy traffic over wifi. Only my android phone experiences some weird issues related to wifi, but it experienced weird issues with my previous archer c7 router as well, so I suspect the phone rather than router. Speeds are a bit lower on DIR-860L wifi compared to my previous Archer c7, but for all other devices it's been rock solid stable, which is most important to me.

@Bartvz

@drbrains

I tried this patch on 2 devices. On my MT7621 I am seeing LAN-WAN speeds around 930Mbps as expected with the sirq maximum at 47%. But if I do a reverse iPerf (only difference adding -R) my WAN - LAN speed is only 250. Without fast path LAN - WAN around 350 so definitely a good improvement to use the patch and WAN-LAN the same around 250. My ISP only gives me 200/20 so no problem, but seems strange to me.

On an MT7628 with only 100M ports, speeds are more or less the same with or without, but SIRQ is about 15% lower with fastpath. Does it still make sense to use it (to reduce system resources used)?

As you can read here, @drbrains has successfully applied fastpat to a mt7621 device. He is using a ZBT WE1326 for that. It might be a nice feature to explore for your builds as well The commit that added the functionality applied cleanly and successfully compiled without any additional modifications. This is the commit in question: https://github.com/dissent1/r7800/commit/93ae487b9e685bd73e6ad301d708009e46f22886

I am a bit short on time, but I will see whether I can apply this commit to the latest master branch once I have time to play around with this