media: au0828: fix null dereference in error path

au0828_usb_disconnect() gets the au0828_dev struct via usb_get_intfdata,
so it needs to set up for the error paths.

Reported-by:  <syzbot+357d86bcb4cca1a2f572@syzkaller.appspotmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>