MFC r196721:

MFC r196721:
  Make sure rx descriptor ring align on 16 bytes. I guess the
  alignment requirement could be multiple of 4 bytes but I think
  using descriptor size would make intention clearer.
  Previously the size of rx descriptor was not power of 2 so it
  caused panic in bus_dmamem_alloc(9).

  Reported by:	Jeff Blank (jb000003 <> mr-happy dot com)
Approved by:	re (kib)

git-svn-id: svn://svn.freebsd.org/base/stable/8@196830 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f