PCI: pciehp: Request control of native hotplug only if supported

Currently we request control of native PCIe hotplug unconditionally.
Native PCIe hotplug events are handled by the pciehp driver, and if it is
not enabled those events will be lost.

Request control of native PCIe hotplug only if the pciehp driver is
enabled, so we will actually handle native PCIe hotplug events.

Suggested-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>