acpi/pcib: Rework the _BBN handling for Host-PCI bridges.

acpi/pcib: Rework the _BBN handling for Host-PCI bridges.

Obtained-from: FreeBSD (Revision 193829 by jhb@freebsd.org)

  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.