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.

Signed-off-by: Le Tan <tamlokveer@gmail.com>