ramips: mt7621: add migration script for WLAN PCI paths

ramips: mt7621: add migration script for WLAN PCI paths

PCI paths of the WLAN devices have changed between kernel 5.10 and 5.15;
migrate config so existing wifi-iface definitions don't break.

This is implemented as a hotplug handler rather than a uci-defaults script
as the migration script must run before the 10-wifi-detect hotplug handler.

based on b452af23a8602ebf7bfb0eb084383ecd595face5

migration was forgotten when device trees were adjusted in
688697889c9ceb9d2d8d413d1abcc5fa7edb402b
c77913be5be5f11a0f20dba1b45b96470099e8a8

fixes #9374

affected devices:
Netgear R6220
Netgear WAC104
Netgear WNDR3700 v5
Zbtlink ZBT-WE1326
Wiflyer WF3526-P
Arcadyan WE420223-99
Beeline Smartbox Flash (Arcadyan WG443223)
MTS WG430223 (Arcadyan WG430223)

Tested-by: Maximilian Baumgartner <aufhaxer@googlemail.com>
Tested-by: Mikhail Zhilkin <csharper2005@gmail.com>
Signed-off-by: Felix Baumann <felix.bau@gmx.de>