PCI: Use "slot" and "pci_slot" for struct hotplug_slot and struct pci_slot

Now in pci_hotplug_core.c, we randomly name a struct hotplug_slot and a
struct pci_slot.  It's easy to confuse them, so let us use "slot" for a
struct hotplug_slot and "pci_slot" for a struct pci_slot.

No functional change.

Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>