V4L/DVB (9165): ivtv: V4L2_FBUF_FLAG_OVERLAY status fix

When the framebuffer format was queried via VIDIOC_G_FBUF,
V4L2_FBUF_FLAG_OVERLAY would only be correctly returned for certain screen
depths.

Signed-off-by: Ian Armstrong <ian@iarmst.demon.co.uk>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>