Retry preloader init when target config is unset

[rohan87674]

Why

Some MT6768/MT6769 devices can successfully respond in BROM/preloader mode before target_config has been populated. In that case, DaHandler.connect() was aborting immediately even though a second preloader.init() call succeeded and recovered the configuration.

This produced a false failure path that required a reconnect even though the device was still valid and usable.

What changed

The connection flow now retries mtk.preloader.init() once when target_config is still None before treating the device as disconnected. The original fail-fast behavior is retained if initialization still does not populate the target configuration.

This keeps the initialization logic aligned with the actual device state and avoids unnecessary disconnect/reconnect cycles for devices that recover immediately.

Validation

• py -3 -m compileall -q mtkclient
• Focused fake-device regression check for the retry path