amd64_edac: fix DRAM base and limit address extraction

K8 DRAM base and limit addresses from F1x40 +8*i and F1x44 + 8*i, where
i in (0..7) are both bits 39-24 and therefore the shifting should be
done by 24 and not by 8.

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