Bluetooth: Shell: Fix unused variable in iso.c

The `iso_qos` was only used for connected ISO, but
was placed outside of the CONFIG_BT_ISO_UNICAST
guard, such that for broadcast ISO-only it was unused.

Move the declaration and renamed to cis_iso_qos.

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