Suspend properly when receiving events without response

In the BTU handler for messages from the controller, rn event may be
received that the stack does not have to respond to. In this case, the
stack should go to sleep.

This patch calls the provided function in the BTU to enter sleep.

Signed-off-by: Jaganath Kanakkassery <jaganathx.kanakkassery@intel.com>
Signed-off-by: Yingkai Cheng <yingkaix.cheng@intel.com>
Signed-off-by: Zhiquan Liu <zhiquan.liu@intel.com>
Change-Id: Ie7c3fcae418eb2480338509e86b7cc870177a965