ixgbe: rewrite ethtool test to use standard config functions

This change makes it so that the ethtool loopback test uses the standard
ring configuration and allocation functions.  As a result the loopback test
will be much more effective at testing core driver functionality.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>