ramips: fix TP-Link mr600 radio partition offset

ramips: fix TP-Link mr600 radio partition offset

This makes 5ghz WiFi work out of the box on these devices, eliminating
the need to flash a magic blob to the radio partition.

This was found by user BulldozerBSG on the OpenWRT Forums:
https://forum.openwrt.org/t/tp-link-archer-mr600-exploration/65489/20
All credit belongs to them. I can confirm the correctness of the
findings. At least one other user (Iggy87100) confirmed them too.

Signed-off-by: Stefan Dösinger <stefandoesinger@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/19790
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>