rc80211-pid: fix adjusted index check

Index: wireless-2.6/net/mac80211/rc80211_pid_algo.c
===================================================================
@@ -78,8 +78,13 @@ static int rate_control_pid_shift_adjust
 	i = j + adj;
 
 	if (i < 0)
+		i = r[0].rev_index;
+	if (i > l - 1)
+		i = r[l - 1].rev_index;
+
+	if (r[i].index <= 0)
 		return r[0].index;
-	if (i >= l - 1)
+	if (r[i].index >= l - 1)
 		return r[l - 1].index;
 
 	tmp = i;