amd64_edac: Fix PCI function lookup

Fix locating sibling memory controller PCI functions by using the
correct PCI domain and use a northbridge descriptor only if found. We
need to at least warn if it wasn't found so that it gets fixed and we
don't go off with wrong results.

Signed-off-by: Daniel J Blueman <daniel@numascale-asia.com>
Link: http://lkml.kernel.org/r/1354265060-22956-1-git-send-email-daniel@numascale-asia.com
[Boris: remove wrong comment, sanitize code and warn if NB desc lookup fails]
Signed-off-by: Borislav Petkov <bp@alien8.de>