MFC r214992:

MFC r214992:
  Reduce spin wait time consumed in GMII register access routine.
  There were a couple of attempts in the past to reduce it since it
  took more than 1ms. Because mii_tick() periodically polls link
  status, waiting more than 1ms for each GMII register access was
  overkill. Unfortunately all previous attempts were failed with
  various ways on different controllers.
  This time, add additional 20us dealy at the end of GMII register
  access which seems to requirement of all RealTek controllers to
  issue next GMII register access request. This is the same way what
  Linux does.

git-svn-id: svn://svn.freebsd.org/base/stable/8@215403 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f