Bluetooth: Controller: Fix assertion using BT_CTLR_LOW_LAT_ULL_DONE

BT_CTLR_LOW_LAT_ULL_DONE is used to reduce CPU usage in LLL
context by delegating the done event dequeue to ULL_HIGH
context.

Building a HCI Controller with BT_CTLR_LOW_LAT_ULL_DONE had
assertion when used with BlueZ stack.

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