dm: core: Remove children before advising uclass

At present the uclass pre-remove method is called before the children are
removed. But the children may refused to be removed, in whch case the
uclass is in a tricky situation. At present we handle this by calling
the uclass' post_probe() method. But it seems better to avoid doing
anything with the uclass in this case.

Switch the ordering so that we make sure the children can be removed
before advising the uclass.

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