Enable -G without --opencl-device

Otherwise, -G doesn't work.

eth/main.cpp

@@ -787,11 +787,14 @@ int main(int argc, char** argv)
 		else if (arg == "-C" || arg == "--cpu")
 			minerType = MinerType::CPU;
 		else if (arg == "-G" || arg == "--opencl")
+		{
 			minerType = MinerType::GPU;
+			miningThreads = 1;
 		/*<< "    -s,--import-secret <secret>  Import a secret key into the key store and use as the default." << endl
 		<< "    -S,--import-session-secret <secret>  Import a secret key into the key store and use as the default for this session only." << endl
 		<< "    --sign-key <address>  Sign all transactions with the key of the given address." << endl
 		<< "    --session-sign-key <address>  Sign all transactions with the key of the given address for this session only." << endl*/
+		}
 		else if ((arg == "-s" || arg == "--import-secret") && i + 1 < argc)
 		{
 			Secret s(fromHex(argv[++i]));

ethminer/main.cpp

@@ -398,7 +398,10 @@ int main(int argc, char** argv)
 		else if (arg == "-C" || arg == "--cpu")
 			minerType = MinerType::CPU;
 		else if (arg == "-G" || arg == "--opencl")
+		{
 			minerType = MinerType::GPU;
+			miningThreads = 1;
+		}
 		else if ((arg == "-D" || arg == "--create-dag") && i + 1 < argc)
 		{
 			string m = boost::to_lower_copy(string(argv[++i]));