net: ethtool: phy: avoid NULL deref when PHY driver is unbound

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

net: ethtool: phy: avoid NULL deref when PHY driver is unbound

phydev->drv can become NULL while the phy_device is still attached to
its net_device, namely after the PHY driver is unbound via sysfs:

echo <mdio_id> > /sys/bus/mdio_bus/drivers/<phy_drv>/unbind

phy_remove() clears phydev->drv but doesn't call phy_detach(), so the
phy_device stays in the link topology xarray and ethnl_req_get_phydev()
still hands it back. ETHTOOL_MSG_PHY_GET then oopses on:

rep_data->drvname = kstrdup(phydev->drv->name, GFP_KERNEL);

drvname is already treated as optional by phy_reply_size(),
phy_fill_reply() and phy_cleanup_data(), so just skip the allocation
when there is no driver bound.