usb: Return -ENOENT when no devices are found

When USB finds no devices it currently returns -EPERM which bootstd does
not understand. This causes other bootdevs of the same priority to be
skipped.

Fix this by returning the correct error code.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>