i40e: Use existing helper to find flow director VSI

Use existing i40e_find_vsi_by_type() to find a VSI
associated with flow director.

Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>