OvmfPkg/PciHotPlugInitDxe: clean up addr. range for non-prefetchable MMIO

OvmfPkg/PciHotPlugInitDxe: clean up addr. range for non-prefetchable MMIO

The non-prefetchable MMIO aperture of a bridge can never fall outside of
the 32-bit address space. Namely, the MemoryBase and MemoryLimit fields in
PCI_BRIDGE_CONTROL_REGISTER have type UINT16, and based on the PCI-to-PCI
Bridge Architecture Spec, Chapter 3.2, the actual MMIO aperture is
determined as in:

NonPrefetchMemoryBase  = (((MemoryBase  & 0xFFF0u) >> 4) << 20) | 0x00000
NonPrefetchMemoryLimit = (((MemoryLimit & 0xFFF0u) >> 4) << 20) | 0xFFFFF

In "OvmfPkg/PciHotPlugInitDxe", the
"mPadding.MmioPadding.AddrSpaceGranularity" field is currently initialized
to 64. According to the above, this is useless generality: a
non-prefetchable MMIO reservation may only be satisfied from 32-bit
address space. Update the field to 32.

In practice this change makes no difference, because PciBusDxe already
enforces the 32-bit limitation when it sees "non-prefetchable" from
(SpecificFlag==0). Quoting commit 8aba40b79267 ("OvmfPkg: add
PciHotPlugInitDxe", 2016-06-30): "regardless of our request for 64-bit
MMIO reservation, it is downgraded to 32-bit".

(See the Platform Init Spec 1.6, Volume 5,
- Table 8. "ACPI 2.0 & 3.0 QWORD Address Space Descriptor Usage", and
- Table 11. "Memory Resource Flag (Resource Type = 0) Usage",
for an explanation of the "mPadding.MmioPadding" fields.)

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Fixes: 8aba40b79267df761bd24d6874ae87f47a7bd3de
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>