Bluetooth: BAP: Broadcast source state checks for ID and BASE

Add state checks for the get_id and get_base functions, as the
ID and base are not valid when the state of the broadcast
source is BT_BAP_EP_STATE_IDLE.

Since the broadcast_source_get_state uses the slist functions,
the broadcast source object for bt_bap_broadcast_source_get_id
can no longer be `const`.

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