amd64_edac: Ensure index stays within bounds in amd64_get_scrub_rate

Add a missing iterator variable thus fixing the conditional of the
for-loop in amd64_get_scrub_rate().

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>