nvme: split devctl out to its own function

Split the devctl aspect of things out to its own function in
nvme_ctrlr_devctl_log. In preparing to document this, and based on
actual use, we want something different for the SMART errors, so this
will facilitate that.

Sponsored by: Netflix
Reviewed by: chuck, mav
Differential Revision: https://reviews.freebsd.org/D44209