PCI: move enum pcie_link_width into pci.h

pcie_link_width is the enum used to define the link width values for a pcie
device. This enum should not be contained solely in pci_hotplug.h, and this
patch moves it next to pci_bus_speed in pci.h

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>