nvme_ctrlr_enable: Small style nits

Rewrite the nested if's using the preferred FreeBSD style for branches
of ifs that return. NFC. Minor tweaks to the comments to better fit new
code layout.

Sponsored by: Netflix
Reviewed by: mav, chuck (prior rev, but comments rolled in)
Differential Revision: https://reviews.freebsd.org/D32245

(cherry picked from commit a245627a4e9553c84eddea07570daaf85c1067b6)