tc: gred: Print usage text if no arguments appear after "gred"

This is more helpful to the user, since the command takes two forms,
and the message that would otherwise appear about missing parameters
assumes one of those forms.
Signed-off-by: David Ward <david.ward@ll.mit.edu>

tc/q_gred.c

@@ -213,6 +213,10 @@ static int gred_parse_opt(struct qdisc_util *qu, int argc, char **argv, struct n
 		argc--; argv++;
 	}
 
+	if (!ok) {
+		explain();
+		return -1;
+	}
 	if (!opt.qth_min || !opt.qth_max || !opt.limit || !avpkt ||
 	    (opt.DP<0)) {
 		fprintf(stderr, "Required parameter (min, max, limit, "