[media] dvb-core/en50221: use dvb_remove_device()

Commit da677fe1 ("[media] dvb-core/en50221: use kref to manage
struct dvb_ca_private") moved the dvb_unregister_device() call to the
kref callback, but that left lots of stale device state visible to
userspace (e.g. in sysfs).  By using dvb_remove_device() and
dvb_free_device() instead of dvb_unregister_device(), we can avoid
that.

Signed-off-by: Max Kellermann <max.kellermann@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>