PCI: Unify pcie_link_speed and pci_bus_speed

These enums must not overlap anyway, since we only have a single
pci_bus_speed_strings array.  Use a single enum, and move it to
pci.h.  Add 'SPEED' to the pcie names to make it clear what they are.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>