usb: xhci: Fix double free on failure

drivers/core/device.c will call `device_free()` after xhci_register
already frees the private device data. This can cause a crash later
during the boot process, observed on aarch64 RPi4b as a synchronous
exception. All callers of xhci_register use priv_auto, so this won't
lead to memory leaks.

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