CVE-2025-38726

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

net: ftgmac100: fix potential NULL pointer access in ftgmac100_phy_disconnect

After the call to phy_disconnect() netdev->phydev is reset to NULL.
So fixed_phy_unregister() would be called with a NULL pointer as argument.
Therefore cache the phy_device before this call.