always have a current_data in a sync group

ebpfcat/ebpfcat.py

@@ -350,6 +350,8 @@ class FastEtherCat(SimpleEtherCat):
 class SyncGroupBase:
     missed_counter = 0
 
+    current_data = None
+
     def __init__(self, ec, devices, **kwargs):
         super().__init__(**kwargs)
         self.ec = ec
@@ -419,8 +421,6 @@ class SyncGroup(SyncGroupBase):
 class FastSyncGroup(SyncGroupBase, XDP):
     license = "GPL"
 
-    current_data = None
-
     properties = ArrayMap()
 
     def __init__(self, ec, devices, **kwargs):