Merge pull request #539 from google/gbg/usb-thread-hotfix

hotfix for usb transport

Author: barbibulle

bumble/transport/usb.py

@@ -139,7 +139,7 @@ def on_packet(self, packet):
             self.packets.put_nowait(packet)
 
         def transfer_callback(self, transfer):
-            self.acl_out_transfer_ready.release()
+            self.loop.call_soon_threadsafe(self.acl_out_transfer_ready.release)
             status = transfer.getStatus()
 
             # pylint: disable=no-member