Clean up in the pci host generic driver

Add clean up on failure and a detach function to the pci host generic
driver.

Reviewed by: jhb (earlier version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35291

(cherry picked from commit d843dd0e1addd2aa69a78be99e251147aafcfd80)