amd64_edac: fix ecc_enable_override handling

amd64_check_ecc_enabled() returns non-zero status when ECC
checking/correcting is disabled and this fails further loading of the
driver even when 'ecc_enable_override' boot param is used.

Fix that by clearing return status in that case.

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