fix(serial): deadlock on tx failure (EPROT-66)

[QSmally]

This patches a deadlock on bus_sema_handle when a tx failure happens, which returns early before releasing the lock.

Tested using the master example on a development board, using injected failures. ESP-IDF v5.4.

Checklist

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.

[CLAassistant]

All committers have signed the CLA.