Fix Linux HID device cache (fixes #595)

ykman/hid/linux.py

@@ -109,15 +109,15 @@ def get_usage(dev):
 
 
 def list_devices():
-    stale = set(_failed_cache)
     devices = []
     for hidraw in glob.glob("/dev/hidraw*"):
-        stale.discard(hidraw)
         try:
             with open(hidraw, "rb") as f:
                 bustype, vid, pid = get_info(f)
                 if vid == YUBICO_VID and get_usage(f) == USAGE_OTP:
                     devices.append(OtpYubiKeyDevice(hidraw, pid, HidrawConnection))
+            if hidraw in _failed_cache:
+                _failed_cache.remove(hidraw)
         except Exception:
             if hidraw not in _failed_cache:
                 logger.debug(
@@ -126,7 +126,4 @@ def list_devices():
                 _failed_cache.add(hidraw)
             continue
 
-    # Remove entries from the cache that were not seen
-    _failed_cache.difference_update(hidraw)
-
     return devices