mdb: fix handling of messages with checksum = 0

mdb/device.py

@@ -35,8 +35,8 @@ class MDBRequest(object):
 
     def validate_checksum(self):
         try:
-            if self.ack:
-                if len(self.data) == 1 and self.processed:
+            if self.processed and self.ack:
+                if len(self.data) == 1:
                     return True  # Only ACK
 
                 return len(self.data) >= 2 and self.data[-2] == compute_checksum(