component/bt : revert the bluetooth library

bluetooth library use a old one, so update to the newest.

diff --git a/components/bt/lib b/components/bt/lib
index e920ec21b8b1bfc0ededb1fde398f846a6fba053..8f6c2c5481d30342be8f69067a68ca7344d7b0c3 160000 (submodule)
--- a/components/bt/lib
+++ b/components/bt/lib
@@ -1 +1 @@
-Subproject commit e920ec21b8b1bfc0ededb1fde398f846a6fba053
+Subproject commit 8f6c2c5481d30342be8f69067a68ca7344d7b0c3