amd64_edac: correct sys address to chip select mapping

The routine does the reverse mapping of the error address of a CECC back
to the node id, DRAM controller and chip select of the DIMM which caused
the error. We should lookup the channel using the syndromes _only_ when
the DCTs are ganged so fix that.

Also, add an early exit when there's an error while scanning for the
csrow thus decreasing indentation levels for better readability.

Finally, fixup comments.

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