Bluetooth: CAP: Always register BAP callbacks with CAP callbacks

Modified bt_cap_stream_ops_register to always register BAP callbacks
to ensure that the callbacks are always forwarded (unless later
overwritten by the application...)

The CAP Initiator Unicast will still register the callbacks itself,
to ensure that the unicast procedures still work even if
bt_cap_stream_ops_register was never called.

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