Re-apply ixgbe unsupported SFP module patch

  commit e473028041ec572dddb62d4f46523ee78044c8e9
  Author: Dan McDonald <danmcd@nexenta.com>
  Date:   Tue Dec 18 17:54:29 2012 +0100

      ixgbe: add ixgbe_allow_unsupported_sfp /etc/system tunable