MdeModulePkg/PciBusDxe: look for the right capability in IsSHPC()

MdeModulePkg/PciBusDxe: look for the right capability in IsSHPC()

The PCI Hot Plug capability register block is marked with capability ID
0x0C (EFI_PCI_CAPABILITY_ID_SHPC), not 0x06
(EFI_PCI_CAPABILITY_ID_HOTPLUG).

This bug prevents PciBusDxe from recognizing whether a PCI-to-PCI bridge
supports hotplug. In turn the platform's EFI_PCI_HOT_PLUG_INIT_PROTOCOL is
not consulted for resource padding information:

  GatherPpbInfo()                [PciEnumeratorSupport.c]
    GetResourcePaddingPpb()      [PciResourceSupport.c]
      GetResourcePaddingForHpb() [PciHotPlugSupport.c]
        IsPciHotPlugBus()        [PciHotPlugSupport.c]
          IsSHPC()               [PciHotPlugSupport.c]
            //
            // returns FALSE
            //
        //
        // the following is not reached:
        //
        gPciHotPlugInit->GetResourcePadding()

Look for the correct capability ID.

Cc: "Johnson, Brian J." <bjohnson@sgi.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Andrew Fish <afish@apple.com>
Cc: Feng Tian <feng.tian@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ruiyu Ni <Ruiyu.ni@intel.com>