igc: Add transmit and receive fastpath and interrupt handlers

This patch adds support for allocating, configuring, and freeing Tx/Rx ring
resources.  With these changes in place the descriptor queues are in a
state where they are ready to transmit or receive if provided buffers.

This also adds the transmit and receive fastpath and interrupt handlers.
With this code in place the network device is now able to send and receive
frames over the network interface using a single queue.

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>