i40e: rework exit flow of i40e_add_fdir_ethtool

Refactor the exit flow of the i40e_add_fdir_ethtool function. Move the
input_label to the end of the function, removing the dependency on
having a non-zero return value. Add a comment explaining why it is ok
not to free the fdir data structure, because the structure is now stored
in the fdir_filter_list.

Change-Id: I723342181d59cd0c9f3b31140c37961ba37bb242
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>