amd64_edac: wrap-up pci config read error handling

Add a pci config read wrapper for signaling pci config space access
errors instead of them being visible only on a debug build. This is
important on amd64_edac since it uses all those pci config register
values to access the DRAM/DIMM configuration of the nodes.

In addition, the wrapper makes a _lot_ (look at the diffstat!) of
error handling code superfluous and improves much of the overall code
readability by removing error handling details out of the way.

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