i7core_edac: return -ENODEV if no MC is found

Nehalem-EX uses a different memory controller. However, as the
memory controller is not visible on some Nehalem/Nehalem-EP, we
need to indirectly probe via a X58 PCI device. The same devices
are found on (some) Nehalem-EX. So, on those machines, the
probe routine needs to return -ENODEV, as the actual Memory
Controller registers won't be detected.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>