[media] v4l2-ioctl: Add a default value for kernel version

Most drivers don't increase kernel versions as newer features are added or
bug fixes are solved. So, vidioc_querycap returned value for cap->version is
meaningless. Instead of keeping this situation forever, let's add a default
value matching the current Linux version.

Drivers that want to keep their own version control can still do it, as they
can override the default value for cap->version.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>