le_print: dump Opcode value and name in the case of LL Control PDU.

This CL adds one line to the textual packet dump (see Opcode: line below):

```
systime=1389684529 freq=2408 addr=9887b821 delta_t=69.836 ms
0f 08 01 0f f8 ff ff 1f 5d 00 0f 7d d3 
Data / AA 9887b821 /  8 bytes
    Channel Index: 2
    LLID: 3 / LL Control PDU
    NESN: 1  SN: 1  MD: 0
    Opcode: 1 / LL_CHANNEL_MAP_REQ

    Data:  01 0f f8 ff ff 1f 5d 00
    CRC:   0f 7d d3
```

I have taken the values from Bluetooth Spec 4.0 [Vol. 6], page 47 of 138.

Let me know if the patch needs to be refined or if it's too tiny to be accepted. I will be happy to fix or extend it.