Bluetooth: Controll: Fix dead code in ll_setup_iso_path

In ll_setup_iso_path cis is only ever set if CONFIG_BT_CTLR_CONN_ISO
is enabled, and similarly adv_stream is only ever set if
CONFIG_BT_CTLR_ADV_ISO is enabled.

The two assignments were reported as dead code by Coverity due
to this, which has been fixed by guarding the code with
the respective Kconfigs.

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