ofw: fix memory leak in ofwbus_attach()

PR: 269509
Reported by: Jaroslaw Pelczar <jarek@jpelczar.com>
Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D38903

(cherry picked from commit 38594ff9c0c9568b5082ba3273103904a6afd38e)