igb: re-use ring configuration code in ethtool testing

Since all of the ring code is now specific to the ring instead of the adapter
struct it is possible to cut a large section of code out of the ethtool
testing configuraiton since we can just use the existing functions to
configure the rings.

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>