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:		36a8572e ("bus_if: provide a default null rescan method")
MFC with:	36a8572e