ixgbe: enable HW RSC for 82599

This patch enables hardware receive side coalescing for 82599 hardware.
82599 can merge multiple frames from the same TCP/IP flow into a single
structure that can span one ore more descriptors.  The accumulated data is
arranged similar to how jumbo frames are arranged with the exception that
other packets can be interlaced inbetween.  To overcome this issue a next
pointer is included in the written back descriptor which indicates the next
descriptor in the writeback sequence.

This feature sets the NETIF_F_LRO flag and clearing it via the ethtool set
flags operation will also disable hardware RSC.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>