Bluetooth: Audio: Add parent service requirement if CSIS cnt > 1

If the number of CSIS instances on a device is
above 1, then each shall be included by another
service.

This creates a bit of a chicken and egg issue,
as we can't really register the "parant" service
before the CSIS instance has been registered,
and we thus can't verify if the parent service
pointer is actually another primary service on this
device. The best we can do is add the proper
type and a NULL check.

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