Bluetooth: controller: df: fix handling of max count of IQ reports

There was an error in handling of max number of IQ reports
generated by controller. Accordin to BT Core Spec 5.1 the host
may request a number of CTEs to be sampled and reported by
controller while enable IQ sampling. The max_cte_count value
set to zero means sample all CTEs in a periodic advertising chain.

The commit fixes wrong handling of the max_cte_count provided
value to generate expected number of IQ reports.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>