Rework the _BBN handling for Host-PCI bridges. Previously we only trusted

Rework the _BBN handling for Host-PCI bridges.  Previously we only trusted
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