pci: pci_host_generic: provide cleanup methods outside of detach

If device_attach() fails, we're expected to actually cleanup after
ourselves because device_detach() will not be called. Factor out the
cleanup bits that don't rely on attach having actually succeeded so
that we can cleanup properly in bcm2838_pci.

Reviewed by: andrew, imp
Differential Revision: https://reviews.freebsd.org/D56896