ADS1115: skip negative values.

src/ads1115.c

@@ -94,6 +94,9 @@ int ads1115_read_conversation_result(const struct ads1115_dev *ads1115,
         return res;
     }
     result->raw = buffer[0] << 8 | buffer[1];
+    if(result->raw & 0x8000) {
+        result->raw = 0;
+    }
     result->voltage = result->raw * VPS;
 
     return 0;