net: ftgmac100: fix potential NULL pointer access in ftgmac100_phy_disconnect

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.