[media] v4l2-ioctl: fix incorrect error code if VIDIOC_DBG_G/S_REGISTER are unsupported

The ioctls VIDIOC_DBG_S_REGISTER and VIDIOC_DBG_G_REGISTER should return -EINVAL
if the driver didn't implement them. Currently they return -EPERM if called as
non-root user. However, this check should only be done if the driver actually
implemented these ioctls. Otherwise, just return -EINVAL as we do with all
unimplemented ioctls.

This bug make the v4l2-compliance test suite fail.

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