igc: Add support for Tx/Rx rings

This change adds the defines and structures necessary to support both Tx
and Rx descriptor rings.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>