media: usb: dvb-usb-v2: zd1301: fix missing platform_device_unregister()

Add the missing platform_device_unregister() before return
from zd1301_frontend_attach in the error handling case when
pdev->dev.driver is empty.

There's an error handling route named err_platform_device_unregister,
so just reuse it.

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>