aioble/security: Only schedule save when needed.

Was getting occasional: RuntimeError: schedule queue full

micropython/bluetooth/aioble/aioble/security.py

@@ -106,6 +106,7 @@ def _security_irq(event, data):
             _secrets[key] = value
 
         # Queue up a save (don't synchronously write to flash).
+        if not _modified:
             _modified = True
             schedule(_save_secrets, None)