dm: core: Handle recursive unbinding of uclass devices

Since a device can have children in the same uclass as itself, we need
to handle unbinding carefully: we must allow that unbinding a device in a
uclass may cause another device in the same uclass to be unbound.

Adjust the code to cope.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>
Tested-by: Joe Hershberger <joe.hershberger@ni.com>