driver core: move struct subsys_dev_iter to a local file

struct subsys_dev_iter is not used by any code outside of
drivers/base/bus.c so move it into that file and out of the global bus.h
file.

Reviewed-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230109175810.2965448-6-gregkh@linuxfoundation.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>