device_attach: Invoke BUS_CHILD_DETACHED if an attach routine fails

This gives the parent bus driver the opportunity to cleanup any
allocated resources, etc. left behind by a failed attach attempt.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D47155

(cherry picked from commit 42078dfb0f72edbf83217cc8c997c0a54d951990)