PCI: acpiphp: Keep driver loaded even if no slots found

Could have root bus hot-added later and there may be slots that need
acpiphp.

The result returned by acpiphp_get_num_slots() is meaningless, because
the bridge the slots are under may be added after this function has been
called, so drop acpiphp_get_num_slots() and the code using it.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>