"Cuckoo30 at 0 gps (graphs per second)" when using cuda and USE_DEVICE = 0

[Felix Weis]

Apparently the plugin doesn't work if in the grin-miner.toml
USE_DEVICE = 0
not commented, even tough ls /dev/nvidia* shows the device as 0th.

The default settings work but not if set explicity.

rust 1.28, grin-miner:master, cuda 9.2, GTX 1080 8GB