Bluetooth: controller: Fix assert on peer unknown rsp

Fixed an assert when peer responded with unknown rsp to
slave feature request when an existing another control
procedure was in progress.

This assert happened with a BT v4.0 peer implementation that
was performing a channel map update and local controller
initiated a slave feature request, receiving an unknown
response.

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