hw/pci-bridge/pxb: Fix missing swizzle

hw/pci-bridge/pxb: Fix missing swizzle

pxb_map_irq_fn() handled the necessary removal of the swizzle
applied to the PXB interrupts by the bus to which it was attached
but neglected to apply the normal swizzle for PCI root ports
on the expander bridge.

Result of this was on ARM virt, the PME interrupts for a second
RP on a PXB instance were miss-routed to #45 rather than #46.

Tested with a selection of different configurations with 1 to 5
RP per PXB instance.  Note on my x86 test setup the PME interrupts
are not triggered so I haven't been able to test this.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Message-Id: <20220118174855.19325-1-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>