hw/mips/fuloong2e: use pci_init_nic_devices()

hw/mips/fuloong2e: use pci_init_nic_devices()

The previous behaviour was: *if* the first NIC specified on the command
line was an RTL8139 (or unspecified model) then it gets assigned to PCI
slot 7, which is where the Fuloong board had an RTL8139. All other
devices (including the first, if it was specified as anything other than
an rtl8319) get dynamically assigned on the bus.

The new behaviour is subtly different: If the first NIC was given a
specific model *other* than rtl8139, and a subsequent NIC was not,
then the rtl8139 (or unspecified) NIC will go to slot 7 and the rest
will be dynamically assigned.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Thomas Huth <thuth@redhat.com>