amd64_edac: fix ECC checking

On the good path of BIOS enabled ECC and no override, the value returned
is 1 by omission and thus is deemed failing by the probe-function.

Allow proper module initialization by clearing the retval explicitly.

Signed-off-by: Doug Thompson <dougthompson@xmission.com>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>