- helpers: fix warning in neutrinoMode_to_string();

cc1plus: warning: function may return address of local variable [-Wreturn-local-addr]
../src/system/helpers.cpp:114:16: note: declared here
    std::string res = s.str();
                ^~~

Signed-off-by: Thilo Graf <dbt@novatux.de>

src/system/helpers.cpp

@@ -111,8 +111,7 @@ const char* neutrinoMode_to_string(int mode)
 			return "NO_REZAP";
 		default:
 			s << std::hex << mode;
-			std::string res = s.str();
-			return res.c_str();
+			return s.str().c_str();
 	}
 }