DMAR: clear dmar_devs[unit] if attach failed

This should stop attempts to use a unit which was not completely
initialized, but referenced by ACPI DMAR table during scoped devices
operions.

PR: 280817
Sponsored by: Advanced Micro Devices (AMD)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D46382