ixgbe: Improve statistics accuracy for DDP traffic

Noticed that the byte and packet count statistics are under-
counting traffic handled by the DDP offload when there is more
than one DDP completion processed in a single call to
ixgbe_clean_rx_irq. This patch fixes that.

I tried to optimize the setting of the rss value so that it
only would have to be computed once, and only when there is
a DDP completion present.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>