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

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

Fixes: a4dd8535a527 ("net: ibm: emac: use devm for register_netdev")
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>