EFI performs its own PCI bus enumeration.  Respect this, and start
controlling devices only when instructed to do so by EFI.

As a side benefit, we should now correctly create multiple SNP
instances for multi-port devices.

This should also fix the problem of failing to enumerate devices
because the PCI bridges have not yet been enabled at the time the iPXE
driver is loaded.

Signed-off-by: Michael Brown <mcb30@ipxe.org>