MFC r228086:

MFC r228086:
  Announce flow control capability to PHY drivers and enable flow
  control for all vr(4) controllers that support it.  It's known that
  old vr(4) controllers(Rhine II) does not support TX pause but Rhine
  III supports both TX and RX pause.
  Make TX pause really work on Rhine III by letting controller know
  available RX buffers.
  While here, adjust XON/XOFF parameters to get better performance
  with flow control.

git-svn-id: svn://svn.freebsd.org/base/stable/9@229641 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f