[media] v4l2-dev: add new VFL_DIR_ defines

These will be used by v4l2-dev.c to improve ioctl checking.
I.e. ioctls for capture should return -ENOTTY when called for
an output device.

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