ivtv: fix incorrect device_caps for ivtvfb

ivtv: fix incorrect device_caps for ivtvfb

The VIDIOC_G_FBUF and related overlay ioctls no longer worked (-ENOTTY was
returned).

The root cause was the introduction of the caps field in ivtv-driver.h.
While loading the ivtvfb module would update the video_device device_caps
field with V4L2_CAP_VIDEO_OUTPUT_OVERLAY it would not update that caps
field, and that's what the overlay ioctls would look at.

It's a bad idea to keep information in two places, so drop the caps field
and only use vdev.device_caps.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reported-by: Martin Dauskardt <martin.dauskardt@gmx.de>
Fixes: bd7d44fc8c7a (media: media/pci: set device_caps in struct video_device)