MdeModulePkg/PciBus: Use actual max bus # for subordinary bus #

MdeModulePkg/PciBus: Use actual max bus # for subordinary bus #

Current code assumes the max bus(0xFF) is under this P2P bridge and
temporarily set it as subordinate bus.
It may cause silicon hangs during PCI enumeration in some specific
case.

Instead, it should get the max bus number from the bus number
resources returned from
PCI_HOST_BRIDGE_RESOURCE_ALLOCATION.StartBusEnumeration() and set it
as subordinate bus.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>