Bluetooth: controller: Fix failing fast encryption setup feature

In commit c41d3edd when implementing the alternative
encryption setup implementation, the original fast
encryption setup implementation was broken. When host is
slow in responding to LTK request, the controller asserted
when fast encryption implementation is selected. This is
now fixed.

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