net: ibm: emac: Fix use-after-free during device removal

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

net: ibm: emac: Fix use-after-free during device removal

The driver was using devm_register_netdev() which causes unregister_netdev()

to be deferred until the devres cleanup phase, which runs after emac_remove()

returns. This creates a use-after-free window where:

1. emac_remove() is called, which tears down hardware (cancels work, detaches

modules, unregisters from MAL)

2. emac_remove() returns

3. devres cleanup runs and finally calls unregister_netdev()

During step 3, the network stack might still process packets, triggering

emac_irq(), emac_poll(), or other handlers that access now-freed hardware

resources (dev->emacp, dev->mal, etc.).

Fix this by replacing devm_register_netdev() with manual register_netdev()

and calling unregister_netdev() at the beginning of emac_remove(), before

any hardware teardown. This ensures the network device is fully stopped and

unregistered before hardware resources are released.

The change is safe because:

- dev->ndev is assigned very early in probe (before any error paths that

could bypass emac_remove)

- platform_set_drvdata() is only called after successful registration, so

emac_remove() only runs for fully registered devices

- unregister_netdev() is idempotent and safe to call on any registered device