ixgbe: Cleanup references to Tx and Rx rings to becommon across the driver

Cleanup all the different references to the Tx ring and Rx ring structures
and make them common across the driver.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
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: Jeff Garzik <jgarzik@redhat.com>