ACPI / hotplug / PCI: Drop WARN_ON() from acpiphp_enumerate_slots()

The WARN_ON() in acpiphp_enumerate_slots() triggers unnecessarily for
devices whose bridges are going to be handled by native PCIe hotplug
(pciehp) and the simplest way to prevent that from happening is to
drop the WARN_ON().

References: https://bugzilla.kernel.org/show_bug.cgi?id=62831


Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>