Extends the link state control feature to CDC-ECM mode.
RNDIS mode prints state changes but doesn't send notifications
to the host yet (would require RNDIS_INDICATE_STATUS_MSG).
For ECM:
- Tracks link state and sends proper CDC notifications
- Handles notification endpoint busy conditions
- Only sends connection notification when link is actually up
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Co-Authored-By: Claude <noreply@anthropic.com>
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