PCI: hotplug: Convert to be builtin only, not modular

Convert CONFIG_HOTPLUG_PCI from tristate to bool.  This only affects
the hotplug core; several of the hotplug drivers can still be modules.

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