To be able to make these APIs usable for apps where the calls may
happen from preemptible tasks, it's safest to return a dedicated
reference and let the caller do bt_conn_put() once its done with the
object.
Change-Id: If227e088385b8c6f61f4061a54c745c0bff7a6c3
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>