OvmfPkg: PciHostBridgeLib: permit access to the full extended config space

By now OVMF makes MdeModulePkg/Bus/Pci/PciHostBridgeDxe go through
MMCONFIG (when running on Q35). Enable the driver to address each B/D/F's
config space up to and including offset 0xFFF.

Cc: Gabriel Somlo <somlo@cmu.edu>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: Michał Zegan <webczat_200@poczta.onet.pl>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Tested-by: Gabriel Somlo <somlo@cmu.edu>
Tested-by: Michał Zegan <webczat_200@poczta.onet.pl>