MEDIUM: counters: use sc0/sc1/sc2 instead of sc1/sc2/sc3

It was a bit inconsistent to have gpc start at 0 and sc start at 1,
so make sc start at zero like gpc. No previous release was issued
with sc3 anyway, so no existing setup should be affected.
diff --git a/include/proto/proto_tcp.h b/include/proto/proto_tcp.h
index 0712764..4644452 100644
--- a/include/proto/proto_tcp.h
+++ b/include/proto/proto_tcp.h
@@ -64,7 +64,7 @@
  */
 static inline int tcp_trk_idx(int trk_action)
 {
-	return trk_action - TCP_ACT_TRK_SC1;
+	return trk_action - TCP_ACT_TRK_SC0;
 }
 
 #endif /* _PROTO_PROTO_TCP_H */