According to the ACPI spec, the bus number of the child PCI bus of a host

to PCI bridge can be read be evaluating the _BBN method of the host to PCI
device.  Unfortunately, there appear to be some lazy/ignorant/moronic/
whatever BIOS writers that return 0 for _BBN for all host to PCI bridges in
the system.  On a system with a single host to PCI bridge this is not a
problem as the child bus of that single bridge will be bus 0 anyway.
However, on systems with multiple host to PCI bridges and l/i/m/w BIOS
writers this is a major problem resulting in all but the first host to
PCI bridge failing to attach.  So, this adds a workaround.

If the _BBN of a host to PCI bridge is zero and pcib0 already exists
and is not us, the we use _ADR to look up our PCI function and slot
(we currently assume we are on bus 0) and use that to call
host_pcib_get_busno() to try and extract our bus number from config
registers on the host to PCI bridge device.  If that fails, then we make
an evil assumption that ACPI's _SB_ namespace lays out the host to PCI
bridges in ascending order and use our pcib unit number as our bus
number.

Approved by:	re

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