CVE-2024-42282 net: mediatek: Fix potential NULL pointer dereference in dummy net_device handling

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

net: mediatek: Fix potential NULL pointer dereference in dummy net_device handling

Move the freeing of the dummy net_device from mtk_free_dev() to
mtk_remove().

Previously, if alloc_netdev_dummy() failed in mtk_probe(),
eth->dummy_dev would be NULL. The error path would then call
mtk_free_dev(), which in turn called free_netdev() assuming dummy_dev
was allocated (but it was not), potentially causing a NULL pointer
dereference.

By moving free_netdev() to mtk_remove(), we ensure it’s only called when
mtk_probe() has succeeded and dummy_dev is fully allocated. This
addresses a potential NULL pointer dereference detected by Smatch[1].