Bluetooth: Controller: Handle LL_UNKNOWN_RSP PDU for remote features

When a peer controller does not recognize one of the LL control
PDUs received, it will issue an LL_UNKNOWN_RSP PDU to let the
peer know that it does not recognize the request.
The controller now handles this incoming PDU and completes the
procedure by issuing the appropriate HCI event in the case of
an LL_FEATURE_REQ and _RSP pair.

Jira: ZEP-1220

Change-Id: I7c04a346441f04deee41198daa6309c11ae1b571
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>