Hack to work around a probe which will lock up at least some i450GX-based
systems.

From the PR:

When 'probe.slot' is PCI_SLOTMAX (== 31) and 'probe.func' is 7,
call to 'pci_cfgread()' here and machine suddenly hangs up.
I don't know why... (or 450GX chipset's bug?)

PR: i386/20379
Submitted by: Masayuki FUKUI <fukui@sonic.nm.fujitsu.co.jp>