PCI/portdrv: Support PCIe services on subtractive decode bridges

The Class Code for subtractive decode PCI-to-PCI bridge is 060401h; add an
entry to make portdrv support this type of bridge.  This allows use of PCIe
services on subtractive decode ports.

Signed-off-by: Honghui Zhang <honghui.zhang@mediatek.com>
[bhelgaas: add braces surrounding entry]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>