V4L/DVB (4585): VIDIOC_G_SLICED_VBI_CAP now accepts a v4l2_buf_type, make it IOWR

The VIDIOC_G_SLICED_VBI_CAP needs to receive the v4l2_buf_type field before
it can return a result. Hence this ioctl must be IOWR, not IOR. Since this
ioctl is still marked experimental we can make this change.

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