bootstd: Fix listing boot devices

bootdev_list() uses uclass_*_device_err() to iterate devices.
However, the only value _err adds is returning an error when the device
pointer is null, and that's checked anyway.

Also there is some intent to report errors, and that's what
uclass_*_device_check() is for, use it.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>