Bluetooth: Fix missing connection cleanup in some scenarios

If the TX thread is not in k_poll() when conn->state gets set to
DISCONNECTED and a dummy buffer is pushed to conn->tx_queue the
bt_conn_prepare_events() function would have failed to add the
connection to the poll list for cleanup. To ensure the cleanup always
happens introduce a new flag that indicates that a cleanup must
happen. The extra benefit of the flag is that we no-longer need a
dummy buffer, but can simply use the conn_change signal to wake up the
TX thread.

Change-Id: I369584d305261ab3666b931c786daff9d131d228
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>