Update to btstack v1.5.6 (#1415)

Fixes #1376
diff --git a/lib/btstack b/lib/btstack
index 0d21232..c51c0a1 160000
--- a/lib/btstack
+++ b/lib/btstack
@@ -1 +1 @@
-Subproject commit 0d212321a995ed2f9a80988f73ede854c7ad23b8
+Subproject commit c51c0a18291602d27cca515530e1baa9239a47ad