i7core_edac: return -ENODEV when devices were already probed

Due to the nature of i7core, we need to probe and attach all PCI
devices used by this driver during the first time probe is called.
However, PCI core will call the probe routine one time for each CPU
socket. If we return -EINVAL to those calls, it would seem that the
driver fails, when, in fact, there's no more devices left to initialize.

Changing the return code to -ENODEV solves this issue.

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