Bluetooth: CSIP: Handle disconnects while in procedure

If a device disconnects while we are in a procedure then
get_next_active_instance would return a service instance pointer
with the `conn` set to NULL.

The issue was caused by the set_info being potentially memset
when the device that disconnected was the one that held the
set_info pointer.
The solution is to not use a pointer, but rather a copy of the
set_info, so that the active.set_info value is still valid
after a disconnect.

Since the set_info is not longer a pointer to a specific
set_info from one of the members, the logs have been updated
as well, as the pointer of the active.set_info is useless
for debugging.

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