iommu/vt-d: fix bug in matching PCI devices with DRHD/RMRR descriptors

Commit "59ce0515

 iommu/vt-d: Update DRHD/RMRR/ATSR device scope
caches when PCI hotplug happens" introduces a bug, which fails to
match PCI devices with DMAR device scope entries if PCI path array
in the entry has more than one level.

For example, it fails to handle
[1D2h 0466   1]      Device Scope Entry Type : 01
[1D3h 0467   1]                 Entry Length : 0A
[1D4h 0468   2]                     Reserved : 0000
[1D6h 0470   1]               Enumeration ID : 00
[1D7h 0471   1]               PCI Bus Number : 00
[1D8h 0472   2]                     PCI Path : 1C,04
[1DAh 0474   2]                     PCI Path : 00,02

And cause DMA failure on HP DL980 as:
DMAR:[fault reason 02] Present bit in context entry is clear
dmar: DRHD: handling fault status reg 602
dmar: DMAR:[DMA Read] Request device [02:00.2] fault addr 7f61e000

Reported-and-tested-by: Davidlohr Bueso <davidlohr@hp.com>
Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>