net/enic: fix crash with removed flow director filters

When flow director filters where removed when an enic device is
stopped, the filters were freed but the pointer was not set to
NULL so the next stop would try to free them again.

Fixes: fefed3d1e62c ("enic: new driver")

Signed-off-by: John Daley <johndale@cisco.com>

drivers/net/enic/enic_clsf.c

@@ -240,6 +240,7 @@ void enic_clsf_destroy(struct enic *enic)
 			vnic_dev_classifier(enic->vdev, CLSF_DEL,
 				&key->fltr_id, NULL);
 			rte_free(key);
+			enic->fdir.nodes[index] = NULL;
 		}
 	}