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.

Reviewed by: manu
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D22213