resource/udev: mark devices from match_subsystem as available

For devices received from match_subsystem, the action attribute is None,
so we need to handle those the same as 'add'.

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>

labgrid/resource/udev.py

@@ -82,7 +82,7 @@ def __attrs_post_init__(self):
 
     def on_device_set(self):
         self.port = self.device.device_node
-        self.avail = self.device.action in ['add', 'change']
+        self.avail = self.device.action in [None, 'add', 'change']
 
 @target_factory.reg_resource
 @attr.s