igc: Add scatter gather support

Scatter gather is used to do DMA data transfers of data that is written to
noncontiguous areas of memory.
This patch enables scatter gather support.

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>