GPU 9999: invalid device ordinal
Hello, 

when I try to start the miner, I get this error.
I've tried with one machine with one Tesla K80 and it worked. 
This test is with multiple K80. (I've tried to add the flag "-d" with values "0" or "0,1" but same issue.

Thanks

Philippe


```
ubuntu@ip-172-31-11-126:~/xmrMiner$ ./xmrMiner -o stratum+tcp://xmr.crypto-pool.fr:3333 -u 41yXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXK -p x --launch=24x39
    *** xmrMiner 0.2.1 (64 bit) for NVIDIA GPUs by psychocrypt
    *** Built with GCC 5.4  using the Nvidia CUDA Toolkit 8.0

 psychocrypt's XMR donation address:
   43NoJVEXo21hGZ6tDG6Z3g4qimiGdJPE6GRxAmiWwm26gwr62Lqo7zRiCJFSBmbkwTGNuuES9ES5TgaVHceuYc4Y75txCTU
 for more donation addresses please read the README.md
-----------------------------------------------------------------

GPU 9999: invalid device ordinal
/home/ubuntu/xmrMinerProject/xmrMiner/cryptonight/cryptonight.cu line 75
```
