nvme_ctrlr_enable: Small style nits

[imp]

Summary

Rewrite the nested if's using the preferred FreeBSD style for branches
of ifs that return. NFC.

Sponsored by: Netflix

[mav]

sys/dev/nvme/nvme_ctrlr.c

318

I have no objections, but if you flip this (en == 1) condition, you could use the same formatting as in next chunk,

[imp]

Start with en == 0, per mav, to make this less indented too.

[chuck]

sys/dev/nvme/nvme_ctrlr.c

306–307

With the restructuring, I'm mixed on whether this comment needs to exist. It was helpful in the else clause to remind the reader of the state. But with the en == 0 immediately above, maybe the comment isn't as useful. I'm good either keeping it or ditching it.

[imp]

Minor tweak in comment (mostly FYI)

[imp]

fold in chuck's comments and go a step further