Bluetooth: Controller: df: Add missing CTE request is_enabled set

There were missing set of is_enabled member of CTE request.
That allowed host to run HCI_LE_Connection_CTE_Request_Enable
command with enable parameter set to true sequently.
That violates BT 5.3 Core specification Vol 4 Part B section
7.8.8.85.

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