samples: Bluetooth: Unicast audio client stream send fix

The sample used `net_buf_ref` to send the same data multiple
times on multiple streams, but the send procedure, which
ends up calling bt_conn_send_cb, does not work
with multiple refs. We thus replace the refs with net_buf_clones.

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