hw/pcie-root-port: Fix PCIe root port initialization

Specify the root port interrupt pin as part of the init
process for cases when msi/msix are not enabled.

Fixes "hw/pci/pci.c:196:23: runtime error: shift exponent -1 is negative"
warning from clang's sanitizer.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
