iommufd/selftest: Don't leak the platform device memory when unloading the module

iommufd/selftest: Don't leak the platform device memory when unloading the module

It should call platform_device_unregister() instead of
platform_device_del() to unregister and free the device.

Fixes: 23a1b46f15d5 ("iommufd/selftest: Make the mock iommu driver into a real driver")
Link: https://lore.kernel.org/r/20230816081318.1232865-1-yangyingliang@huawei.com
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>