Bluetooth: ISO: Cleanup BIG before stopped callback

Modify the big_disconnect function to do cleanup_big before
calling the `stopped` callback. This will allow the ISO channels,
as well as the BIG itself, to be reused for other purposes directly
in the stopped callback, without having to offload such actions
to a separate thread.

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