V4L/DVB (12722): v4l2-dev: replace 'kernel number' by 'device node number'.

The term 'kernel number' is very vague, so replace it with the somewhat more
descriptive term 'device node number'.

In one place the local variable 'nr' was used to create the device node number
of the new device name. This has been replaced with the vdev->num field to
more clearly mark this as being the device node number and not the minor
number.

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