amd64_edac: add error decoding logic

Borislav:

- fold amd64_error_info_valid() into its only user
- fix/cleanup comments
- fix function return value patterns
- cleanup debug calls
Reviewed-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Signed-off-by: Doug Thompson <dougthompson@xmission.com>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>