usb-device-mtp: Improve string handling and add conditional debug logging

- Add helper method _write_mtp_string for consistent UTF-16 string formatting
- Add debug flag to enable/disable verbose logging (default: off)
- Convert print statements to use the new _log helper method
- Improve large file transfer handling with better buffer management
- Add progress reporting for large file transfers
- Update example to demonstrate debug flag usage

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: andrewleech

micropython/usb/examples/device/mtp_example.py

@@ -24,7 +24,8 @@
 import time
 
 # Create an MTP interface that exposes the root directory
-mtp = MTPInterface(storage_path="/")
+# Set debug=True to enable detailed logging
+mtp = MTPInterface(storage_path="/", debug=True)
 
 # Initialize the USB device with the MTP interface
 # Keep builtin_driver=True to maintain the serial REPL alongside MTP