ixgbe: Do not use DCA to prefetch the entire packet into the cache

The way the code was previously written it was causing DCA to prefetch the
entire packet into the cache when it was enabled.  That is excessive as we
only really need the headers.

We are now prefetching the headers via software so doing this from DCA would
be redundant anyway.  So clear the bit that was causing us to prefetch the
packet data and instead only use DCA for the descriptor rings.

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