ACPI PCI hotplug: harden against panic regression

ACPI hotplug panic with current git head
http://lkml.org/lkml/2009/1/10/136



Rather than reverting the entire commit that causes the crash:
e8c331e9
"PCI hotplug: introduce functions for ACPI slot detection"

simply harden against it while the changes to
the hotplug code on this particularl machine are understood.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Len Brown <len.brown@intel.com>