Bluetooth: controller: legacy: Fix Tx Ctrl PDU leak

Overlapping Feature Exchange requested by host with
Encryption Setup requested by the application caused the
controller to corrupt its Tx queue leading to Tx Ctrl PDU
buffers from leaking from the system.

Relates to #21299.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>