igb: transition driver to only using advanced descriptors

Currently the driver uses advanced descriptors for its main functionality,
but then uses legacy when testing.  This patch changes this so that
advanced descriptors are used throughout and all mentions of legacy
descriptors are removed.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>