Bluetooth: BAP: Replace bt_bap_iso with bt_iso_chan for streams

Modify the struct bt_bap_stream to have a direct reference to the
bt_iso_chan, rather than having a reference to the parent
bt_bap_iso.

This change will allow users to more easily and directly access
the bt_iso_chan, instead of getting the reference via
bt_bap_ep_get_info.
Additionally, this allows users to access the bt_iso_chan for
streams that are not codec configured, and thus not have a
bt_bap_ep reference.

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