a _BBN value of 0 if it was for the first bridge encountered since some
older systems returned _BBN of 0 for all bridges. However, some newer
systems enumerate bridges with non-zero _BBN before bus 0 which is
perfectly valid. Handle both cases by trusting the first bridge that has
a _BBN of 0 and falling back to reading from non-standard config registers
only for subsequent bridges with a _BBN of 0. We also only perform this
check for segment (domain) 0. We assume that _BBN is always correct
for segments other than 0.
Tested by: Josef Moellers josef.moellers at fujitsu
MFC after: 1 week
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