Bluetooth: ISO: Change bis_id to bis_number

Modify the bis_id in struct bt_conn_iso to be bis_number
and correctly initialize it for both the broadcaster and
the sync receiver.

The field is currently unused and the term bis_id does not
exist in the core spec. The BIS Index was renamed to BIS
Number in Core 6.1, and thus the Zephyr code base will
reflect that.

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