Don't add suffix if user added it already

```
$ ssh-tpm-keygen
Generating a sealed public/private ecdsa key pair.
Enter file in which to save the key (/home/jc/.ssh/id_ecdsa): /home/jc/.ssh/hh-8192_ecdsa.tpm
Enter pin (empty for no pin):
Confirm pin:
Your identification has been saved in /home/jc/.ssh/hh-8192_ecdsa.tpm.tpm
```

Expected. `Your identification has been saved in /home/jc/.ssh/hh-8192_ecdsa.tpm`