[pci] Generalise pci_num_bus() to pci_discover()

[pci] Generalise pci_num_bus() to pci_discover()

Allow pci_find_next() to discover devices beyond the first PCI
segment, by generalising pci_num_bus() (which implicitly assumes that
there is only a single PCI segment) with pci_discover() (which has the
ability to return an arbitrary contiguous chunk of PCI bus:dev.fn
address space).

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