e1000e: 82571 SerDes link handle null code word from partner

SerDes Link detection on certain 82571 mezzanine cards can fail when the
link is forced, the link partner does not recognize forced link and the
link partner sends null code words.  Detect the null code words and return
to auto-negotiation state which causes the link partner to begin responding
with valid code words.  Within a reasonable interval the link will finally
settle as forced by both partners.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>