amd64_edac: do not enable module by default

While at it, fix a link failure when !K8_NB.

Acked-by: Doug Thompson <dougthompson@xmission.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Tested-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>