Silicon/AMD/Styx/PlatInitPei: ignore bogus MAC addresses provided by the SCP

In some cases, the SCP on B0 Overdrive returns all ones for the
Ethernet MAC addresses, which breaks networking not only in UEFI
but also in the OS. So ignore the SCP in this case, and keep the
MAC addresses that have already been set at build time.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Leif Lindholm <leif.lindholm@linaro.org>