lspci: Decode "Slot Implemented" for PCI/PCI-X to PCIe Bridges

lspci: Decode "Slot Implemented" for PCI/PCI-X to PCIe Bridges

The secondary side of a PCI/PCI-X to PCIe Bridge (a "reverse bridge") is a
PCIe Downstream Port and could support a slot just like Root Ports and
Switch Downstream Ports.

Decode "Slot Implemented" for reverse bridges and, if true, the Slot
Capabilities, Control, and Status registers.

For a reverse bridge with no slot, the difference in the "lspci -vv" output
looks like this:

-    Capabilities: [40] Express (v2) PCI/PCI-X to PCI-Express Bridge, MSI 00
+    Capabilities: [40] Express (v2) PCI/PCI-X to PCI-Express Bridge (Slot-), MSI 00

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>