net: phy: don't try to setup PHY-driven SFP cages when using genphy

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

net: phy: don't try to setup PHY-driven SFP cages when using genphy

We don't have support for PHY-driver SFP cages with the genphy code.

On top of that, it was found by sashiko that running

sfp_bus_add_upstream() for genphy deadlocks, as for genphy the PHY

probing runs under RTNL, which isn't the case for non-genphy drivers.

This problem was reproduced, and does lead to a deadlock on RTNL.

Before the blamed commit, the phy_sfp_probe() call was made by

individual PHY drivers, so there was no way to get to the SFP probing

path when using genphy.

Let's therefore only run phy_sfp_probe when not using genphy.