MdeModulePkg/PciBus: Should reserve enough bus number for HPC

MdeModulePkg/PciBus: Should reserve enough bus number for HPC

When there is no bridge before the HPC (hot plug controller),
the issue cannot be seen.
But when there are bridges before the HPC, the PciBus will only
use the value (= <CurrentBusNumber> + <ReservedBusNumber>) as the
sub-ordinary bus number for HPC.
The correct sub-ordinary bus number should be:
<CurrentBusNumber> + <OccupiedBusNumber>(by earlier bridges) +
<ReservedBusNumber>.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>