efi_loader: fix device-path for USB devices

EFI device paths for block devices must be unique. If a non-unique device
path is discovered, probing of the block device fails.

Currently we use UsbClass() device path nodes. As multiple devices may
have the same vendor and product id these are non-unique. Instead we
should use Usb() device path nodes. They include the USB port on the
parent hub. Hence they are unique.

A USB storage device may contain multiple logical units. These can be
modeled as Ctrl() nodes.

Reported-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>