Check TLP packet successful completion status.  This fix the issue when
accessing multi-function devices in enumeration process, TLP will return
error when accessing non-exist function number.  Returns PCI error code
instead of generic errno.

Tested on Ethernet adapter card with multi-functions.

[bhelgaas: simplify completion status checking code]
Signed-off-by: Ley Foon Tan <lftan@altera.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>