amd64_edac: cleanup amd64_check_ecc_enabled

Simplify code flow and make sure return value is always valid since
further driver init depends on it. Carve out long warning string and
make code more readable. Shorten some names, while at it.

There should be no functional change resulting from this patch.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>