bgpd: Use correct encoding before printing shutdown msg

bgpd: Use correct encoding before printing shutdown msg

Using `bgp shutdown message MSG...`.

Length should be decoded from the first byte, but it's decoded from the data
instead.

Before:

```
%NOTIFICATION: sent to neighbor 192.168.0.2 6/2 (Cease/Administratively Shutdown) 70 bytes 5b 54 49 43 4b 45 54 2d 31 2d 31 34 33 38 33 36 37 33 39 30 5d 20 73
```

After:

```
%NOTIFICATION: sent to neighbor 192.168.0.2 6/2 (Cease/Administratively Shutdown) "[TICKET-1-1438367390] software upgrade; Expected downtime for 2 hours;"
```

On receiving side:

```
"[TICKET-1-1438367390] software upgrade; Expected downtime for 2 hours;"
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>