media: v4l2-ioctl.c: readbuffers depends on V4L2_CAP_READWRITE

If V4L2_CAP_READWRITE is not set, then readbuffers must be set to 0,
otherwise v4l2-compliance will complain.

A note on the Fixes tag below: this patch does not really fix that commit,
but it can be applied from that commit onwards. For older code there is no
guarantee that device_caps is set, so even though this patch would apply,
it will not work reliably.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: 049e684f (media: v4l2-dev: fix WARN_ON(!vdev->device_caps))
Cc: <stable@vger.kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>