amd64_edac: Simplify ECC override handling

No need for clearing ecc_enable_override and checking it in two places.
Instead, simply check it during probing and act accordingly. Also,
rename the flag bitfields according to the functionality they actually
represent. What is more, make sure original BIOS ECC settings are
restored when the module is unloaded.

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