[efi] Fall back to assuming identity mapping of MMIO address space

[efi] Fall back to assuming identity mapping of MMIO address space

Some UEFI systems (observed with a Supermicro X11SPG-TF motherboard)
seem to fail to provide a valid ACPI address space descriptor for the
MMIO address space associated with a PCI root bridge.

If no valid descriptor can be found, fall back to assuming that the
MMIO address space is identity mapped, thereby matching the behaviour
prior to commit 27e886c ("[efi] Use address offset as reported by
EFI_PCI_ROOT_BRIDGE_IO_PROTOCOL").

Debugged-by: Tore Anderson <tore@fud.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>