cxl: fix possible null-ptr-deref in cxl_guest_init_afu|adapter()

In the Linux kernel, the following vulnerability has been resolved:

cxl: fix possible null-ptr-deref in cxl_guest_init_afu|adapter()

If device_register() fails in cxl_register_afu|adapter(), the device
is not added, device_unregister() can not be called in the error path,
otherwise it will cause a null-ptr-deref because of removing not added
device.

As comment of device_register() says, it should use put_device() to give
up the reference in the error path. So split device_unregister() into
device_del() and put_device(), then goes to put dev when register fails.