Bringing support for realtek-poe to mainline OpenWrt

@Borromini You reported here that realtek-poe on your 8-port Realtek + Broadcom-based switches stopped functioning.

@mrnuke reports that:

hurricos: I have to send these commands to the PoE ctrlr on the EWS2910P switch: https://paste.debian.net/1241353/

I'd like to work with you to clean up realtek-poe a bit further to make sure it works on the 8-port variants (I don't have any), then to test what sending that linked block above as such would do:

realtek-poe does not continue to send configuration after the initial setup, so you can actually send these frames using realtek-poe mrnuke: You should use my patches in my tree: https://github.com/hurricos/openwrt/tree/realtek-poe

Then, rework this into e.g. ubus call poe sendframe '{"frame": "0x09 0x01"}' -- that's the first frame, you are skipping the frame number, padding and checksum at the end. PLEASE let me know if this works after compiling my realtek-poe for your system.

I would be VERY interested to hear if this DOES NOT work; in particular, you can stop the realtek-poe service on your own and run /etc/bin/realtek-poe -d and collect the output.

To that last bit -- I think there is an issue with my realtek-poe -- I do iterate at one point over MAX_PORTS and not 8. I should fix this to use the currently discovered number of ports, but I don't know if it'd crash things -- @mrnuke will be giving me their crash report at some point.