Align the PCI Express #defines with the style used for the PCI-X
#defines.  This has the advantage that it makes the names more
compact, and also allows us to correct the non-uniform naming of
the PCIM_LINK_* defines, making them all consistent amongst themselves.

This is a mostly mechanical rename:
  s/PCIR_EXPRESS_/PCIER_/g
  s/PCIM_EXP_/PCIEM_/g
  s/PCIM_LINK_/PCIEM_LINK_/g

In this MFC, #defines have been added for the old names to assist
merges and out-of-tree drivers.