software/usb/device: endpoint should keep reference to the usb device

without reference to the device, and depending on the invocation
the usb device may be collected by the garbage collector while
the usb endpoint is still in use.

this causes the following error which is difficult to trace:
python: core.c:1278: libusb_ref_device: Assertion `refcnt >= 2' failed.

now the endpoint keeps a reference to the device preventing it to be freed

lambdalib/software/usb/device.py

@@ -7,13 +7,15 @@
 
 
 class USBEndpoint():
-    def __init__(self, handle, size, num, asynchronous=True, context=None, timeout=1000):
+    def __init__(self, handle, size, num, asynchronous=True, context=None,
+                 timeout=1000, device=None):
         self.handle = handle
         self.size = size
         self.num = num
         self.asynchronous = asynchronous
         self.context = context
         self.timeout = timeout
+        self.device = device
 
         # function alias
         self.write = self.send
@@ -98,7 +100,8 @@ def control_read(self, *args):
 
     def get_endpoint(self, num, asynchronous=True):
         return USBEndpoint(self.handle, self.bulksize, num,
-                           asynchronous=asynchronous, context=self.context)
+                           asynchronous=asynchronous, context=self.context,
+                           device=self)
 
     def __del__(self):
         if self.handle: