dev_refthread: Do not initialize *ref when reference was not acquired

Like the companion API devvn_refthread, leave *ref uninitialized when a
reference was not acquired. Initializing to 1 provides a vaguely
correct-looking but bogus value for broken callers to (mistakenly) pass to
dev_relthread() when refthread fails.

Make it even more clear to consumers that dev_relthread is only valid when
dev_refthread succeeds.

Reviewed by: kib, markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D16885