There is an existing helper method in subr_bus.c, but almost no drivers
know to use it. It also returns the same error as an empty method,
making it not very useful. Move this to bus_if.m and return a more
sensible error code.

This gives a slightly more meaningful error message when attempting
'devctl rescan' on buses and devices alike:
  "Device not configured" --> "Operation not supported by device"

Reviewed by:	imp
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D35501