hw/pci-host/ppc440_pcix: Do not expose a bridge device on PCI bus

hw/pci-host/ppc440_pcix: Do not expose a bridge device on PCI bus

Real 460EX SoC apparently does not expose a bridge device and having
it appear on PCI bus confuses an AmigaOS file system driver that uses
this to detect which machine it is running on.

Cc: qemu-stable@nongnu.org
Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240411192443.B4D644E6026@zero.eik.bme.hu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
(cherry picked from commit 6e4aceba2079e3df42edc89d44f4ee02343bb09e)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>