pci: assign devfn to pci_dev before calling pci_device_iommu_address_space()

In function do_pci_register_device() in file hw/pci/pci.c, move the assignment
of pci_dev->devfn to the position before the call to
pci_device_iommu_address_space(pci_dev) which will use the value of
pci_dev->devfn.

Fixes: 9eda7d37
    pci: Introduce helper to retrieve a PCI device's DMA address space

Cc: qemu-stable@nongnu.org
Signed-off-by: Le Tan <tamlokveer@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit efc8188e)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>