i40e: drop redundant check when setting xdp prog

Net core handles the case where netdev has no xdp prog attached and
current prog is NULL. Therefore, remove such check within
i40e_xdp_setup.

Reviewed-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: Kiran Bhandare <kiranx.bhandare@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>