amd64_edac: Correct scrub rate setting

Exit early when setting scrub rate on unknown/unsupported families.

Cc: <stable@kernel.org> # 32.x 33.x 34.x
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
Acked-by: Doug Thompson <dougthompson@xmission.com>