i40e: Cleanup in FDIR SB ethtool code

Function add_del_fdir was used and implemented only for add. So change the name
and drop a parameter.

Change-ID: Icf2c6c3bbd4fd00cf8d9613a3f6d8c08e0f8e288
Signed-off-by: Anjali Singhai Jain <anjali.singhai@intel.com>
Signed-off-by: Catherine Sullivan <catherine.sullivan@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>