Bluetooth: BAP: Modify unicast client callbacks to slist

Modify the BAP unicast client callback structure to be a
linked list. The purpose of this is to have multiple listeners
of the unicast client changes and notifications.
This is needed for the CAP initiatior.

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

include/zephyr/bluetooth/audio/bap.h

@@ -1462,6 +1462,11 @@ struct bt_bap_unicast_client_cb {
 	 * If discovery procedure has complete both @p codec and @p ep are set to NULL.
 	 */
 	void (*discover)(struct bt_conn *conn, int err, enum bt_audio_dir dir);
+
+	/** @cond INTERNAL_HIDDEN */
+	/** Internally used field for list handling */
+	sys_snode_t _node;
+	/** @endcond */
 };
 
 /**
@@ -1474,7 +1479,7 @@ struct bt_bap_unicast_client_cb {
  *
  * @return 0 in case of success or negative value in case of error.
  */
-int bt_bap_unicast_client_register_cb(const struct bt_bap_unicast_client_cb *cb);
+int bt_bap_unicast_client_register_cb(struct bt_bap_unicast_client_cb *cb);
 
 /**
  * @brief Discover remote capabilities and endpoints