subr_bus: restore bus_null_rescan()

Partially revert the previous change; we need to keep this method as a
specific override for pci_driver subclasses which should not use
pci_rescan_method() -- cardbus and ofw_pcibus. However, change the return
value to ENODEV for the same reasoning given in the original commit, and
use this as the default rescan method in bus_if.m.

Reported by: jhb
Fixes: 36a8572ee8f5 ("bus_if: provide a default null rescan method")
MFC with: 36a8572ee8f5