igc: Add netdev

Now that we have the ability to configure the basic settings on the device
we can start allocating and configuring a netdev for the interface.

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>