V4L/DVB: v4l2-dev: after a disconnect any ioctl call will be blocked

Until now all fops except release and (unlocked_)ioctl returned an error
after the device node was unregistered. Extend this as well to the ioctl
fops. There is nothing useful that an application can do here and it
complicates the driver code unnecessarily.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>