In the nexus case, there are no ivars for children of nexus devices,
and we were passing data in from before the device existed, hence ivars
are convenient as the softc doesn't really exist yet.
However, for pci->pci bridges, the pcib occupies a pci device itself,
which *does* already have ivars. However, softc is available and stable
at this point since we've been identified and are locating the bus during
attach. So, use softc for this version of pcib devices for storing the
physical bus number in.
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