Bluetooth: controller: Fix handling of invalid Ctrl PDU lengths

Fix the controller implementation to handle Control PDUs
with invalid lengths by responding with Unknown Response
PDU.

Fixes LL.TS.5.0.2 conformance tests:
LL/PAC/SLA/BI-01-C [Control PDUs with Invalid Length from
                    Master]
LL/PAC/MAS/BI-01-C [Control PDUs with Invalid Length from
                    Slave]

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