dm: usb: Convert 'usb' command to support driver model

Adjust this command to work with the new driver model uclass. It needs to
iterate through multiple independent controllers to find hubs, and work
through their children recursively in a different way. Otherwise the
functionality is much the same.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>