Bluetooth: controller: Add validation of received LLCP PDUs

This adds suport for validation of received PDUs before they are
pass further to LLCP state machines. If PDU size is invalid it is
rejected with LL_UNKNOWN_RSP.

This was affecting following qualification test cases:
LL/PAC/PER/BI-01-C
LL/PAC/CEN/BI-01-C

Signed-off-by: Szymon Janc <[email protected]>

Author: sjanc