BUILD: stream-int: avoid a build warning in dev mode in si_state_bit()

The BUG_ON() test emits a warning about an always-true comparison regarding
<state> which cannot be lower than zero. Let's get rid of it.
diff --git a/include/proto/stream_interface.h b/include/proto/stream_interface.h
index 29eda6b..50373ac 100644
--- a/include/proto/stream_interface.h
+++ b/include/proto/stream_interface.h
@@ -143,7 +143,7 @@
 /* returns a bit for a stream-int state, to match against SI_SB_* */
 static inline enum si_state_bit si_state_bit(enum si_state state)
 {
-	BUG_ON(state < SI_ST_INI || state > SI_ST_CLO);
+	BUG_ON(state > SI_ST_CLO);
 	return 1U << state;
 }