realtek: EnGenius EWS2910P: use the mtd3 partition for root overlay

[Russell Senior]

Back in 2022, 7bba7ccde9cb021e24ec83f1332aafd5898e23ef was committed
to change the partition used for rootfs_data on the EnGenius EWS2910P.

After sitting on the shelf for a long time, I just got my EWS2910P v3
out and was testing the new PSE control infrastructure, and noticed
that sysupgrade -v -n <foo> was not clearing the overlay filesystem as
expected. I think this patch may be responsible. I can, post install,
do a factory reset and the overlay filesystem is then cleared.

Anyone have suggestions for how to fix this?

-- 
Russell Senior
russell at personaltelco.net