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>
diff --git a/common/cmd_usb.c b/common/cmd_usb.c
index ca06826..2ed4cb4 100644
--- a/common/cmd_usb.c
+++ b/common/cmd_usb.c
@@ -630,12 +630,11 @@
 		     bus;
 		     uclass_next_device(&bus)) {
 			struct usb_device *udev;
-			struct udevice *hub;
+			struct udevice *dev;
 
-			device_find_first_child(bus, &hub);
-			if (device_get_uclass_id(hub) == UCLASS_USB_HUB &&
-			    device_active(hub)) {
-				udev = dev_get_parentdata(hub);
+			device_find_first_child(bus, &dev);
+			if (dev && device_active(dev)) {
+				udev = dev_get_parentdata(dev);
 				usb_show_tree(udev);
 			}
 		}