PCI: checking busn_res in pci_scan_root_bus()

Some callers do not supply the bus number aperture, usually because they do
not know the end.  In this case, we assume the aperture extends from the
root bus number to bus 255, scan the bus, and shrink the bus number
resource so it ends at the largest bus number we found.

This is obviously not correct because the actual end of the aperture may
well be larger than the largest bus number we found.  But I guess it's all
we have for now.

Also print out one info about that, so we could find out which path
does not have busn_res in resources list.

[bhelgaas: changelog, _safe iterator unnecessary, use %pR format for bus]
Signed-off-by: Yinghai Lu <yinghai@kernel.org>