hw/ppc/prep: use pci_init_nic_devices()

hw/ppc/prep: use pci_init_nic_devices()

Previously, the first PCI NIC would be placed in PCI slot 3 and the rest
would be dynamically assigned. Even if the user overrode the default NIC
type and made it something other than PCNet.

Now, the first PCNet NIC (that is, anything not explicitly specified
to be anything different) will go to slot 3 even if it isn't the first
NIC specified on the commnd line. And anything else will be dynamically
assigned.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>