MFC 309588: Don't attach to Host-PCI bridges with a bad bus number.

If the bus number assigned to a Host-PCI bridge doesn't match the first
bus number in the associated producer range from _CRS, print a warning and
fail to attach rather than panicking due to an assertion failure.

At least one single-socket Dell machine leaves a "ghost" Host-PCI bridge
device in the ACPI namespace that seems to correspond to the I/O hub in
the second socket of a two-socket machine. However, the BIOS doesn't
configure the settings for this "ghost" bridge correctly, nor does it have
any PCI devices behind it.