ixgbe: rename autoneg variables

Renames some autoneg/speed variables to be more consistent with check_link,
get_link_capabilities, and setup_link function calls. Initializes instances
of autoneg.

Signed-off-by: Josh Hay <joshua.a.hay@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>