dm: usb: Do not assume that first child is always a hub

On some single port (otg) controllers there is no emulated root hub, so
the first child (if any) may be one of: UCLASS_MASS_STORAGE,
UCLASS_USB_DEV_GENERIC or UCLASS_USB_HUB.

All three of these (and in the future others) are suitable for our
purposes, remove the check for the device being a hub, and add a check to
deal with the fact that there may be no child-dev.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Simon Glass <sjg@chromium.org>