V4L/DVB: v4l2-framework.txt: fix incorrect statement

After unregister_device all fileops are blocked, except for ioctls.
So it is not just the open that is blocked, others are as well.

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