The logical pci busses must attach to the physical pci bridges using the

The logical pci busses must attach to the physical pci bridges using the
same pci identifier used for the bridge.  If a machine has multiple
motherboard PCI bridges and also has a pci-to-pci bridge hanging off one
of those bridges, it was possible for the pci-to-pci bridge to be handed
an identifier for a motherboard pci bridge bus number which has not yet
been probed.  Then, later on, when the motherboard pci bridge bus is
probed the system believes that it has already been probed and refuses to
scan it.

This is why pcib #1, which gets added *AFTER* pcib #2 on a DELL 2550 does
not appear to exist on a DFly or FreeBSD-4 box.  pcib #1 is what the
BGE network driver is hanging off of on a DELL 2550.

This commit uses the secondary bus identifier as the starting unit number
for the attach rather then -1 (which scans forward from unit 0).  I am
not entirely sure that this is the correct fix.