urtwn: fix A-MSDU tx aggregation when encryption is used.

Clear unused (undocumented) CAM bytes while setting a key;
without that, hardware does weird things when A-MSDU bit in QoS header
is set.

Tested with RTL8188CUS (AP) -> RTL8188EU (STA) (A-MSDU transmit).

Reported by: many
Obtained from: https://github.com/s3erios/urtwm
MFC after: 5 days