ixgbe: Use CONFIG_XFRM_OFFLOAD instead of CONFIG_XFRM

There is no point in adding code if CONFIG_XFRM is defined that we won't
use unless CONFIG_XFRM_OFFLOAD is defined. So instead of leaving this code
floating around I am replacing the ifdef with what I believe is the correct
one so that we only include the code and variables if they will actually be
used.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Shannon Nelson <shannon.nelson@oracle.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>