Bluetooth: Audio: VCS register return if already registered

Change how bt_vcs_register works if VCS has already been
registered.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>

include/bluetooth/audio/vcs.h

@@ -81,6 +81,7 @@ struct bt_vcs_included {
  *
  * @param      param  Volume Control Service register parameters.
  * @param[out] vcs    Pointer to the registered Volume Control Service.
+ *                    This will still be valid if the return value is -EALREADY.
  *
  * @return 0 if success, errno on failure.
  */