dm: core: Correct bug introduced in uclass_first/next_device()

These functions now rely on uclass_find_first/next_device() and assume that
they will either return failure (-ve error code) or a device. In fact,
coming to the end of a list is not considered failure and they return 0
in that case.

The logic to deal with this was replaced in commit acb9ca2a with just using
uclass_get_device_tail(). Add back the missing logic. This bug was
caught by unit tests but since they were broken for other reasons at the
time, this was not noticed.

Signed-off-by: Simon Glass <sjg@chromium.org>