ixgbe: Remove legacy descriptor support

The ethtool offline test is the only consumer of the legacy descriptors.
Update that path to only use advanced descriptors, and remove all support
for legacy descriptors.

Signed-off-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>