dm: core: Switch uclass_foreach_dev_probe to use simple iterator

The return value is not used for anythig, and in a later patch the
behavior of the _err iterator will change in an incompatible way.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Update pvblock_probe() to avoid using internal var:
Signed-off-by: Simon Glass <sjg@chromium.org>