MFC r356649: regulator: small enhancements to regulator_shutdown
Highlights:
- Exit early if we're not disabling unused regulators; there's no need to take the regulator topology lock and re-evaluate this every iteration, as it's not going to change.
- Don't emit a notice that we're shutting down a regulator if it's not enabled, to reduce noise.
- Mention the outcome of the shutdown, to aide debugging and easily let developer/user collect list of regulators we actually shutdown to determine problematic one.