CVE-2026-98620: net: ethernet: mtk_eth_soc: unregister net_devices in case of probe failure

In the Linux kernel, the following vulnerability has been resolved:

net: ethernet: mtkethsoc: unregister netdevices in case of probe failure

If registernetdev() fails for one of the MTKMAXDEVS devices in mtkprobe(), the error path jumps to errdeinitppe, skipping mtkunregdev(). The previously registered netdevices are then freed by mtkfreedev() while still in NETREGREGISTERED state, hitting the BUGON(dev->regstate != NETREGUNREGISTERED).

Route the registernetdev() failure to errunregnetdev so the netdevices registered so far are properly unregistered before being freed.