PCI: Add slot reset option to pci_dev_reset()

If the hotplug controller provides a way to reset a slot, use that
before a direct parent bus reset.  Like the bus reset option, this is
only available when a single pci_dev occupies the slot.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>