MFC r209865:

MFC r209865:
  Some revision of Yukon controller generates corrupted frame when TX
  checksum offloading is enabled.  The frame has a valid checksum
  value so payload might be modified during TX checksum calculation.
  Disable TX checksum offloading but give users chance to enable it
  when they know their controller works without problems with TX
  checksum offloading.

  Reported by:	Andrzej Tobola <ato <> iem dot pw dot edu dot pl>

git-svn-id: svn://svn.freebsd.org/base/stable/8@211359 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f