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.