[media] em28xx: Fix vidioc fmt vid cap v4l2 compliance

Set fmt.pix.priv to zero in vidioc_try_fmt_vid_cap.
Catched by v4l2-compliance.

Signed-off-by: Alban Browaeys <prahal@yahoo.com>
[hans.verkuil@cisco.com: dropping unnecessary change to g_fmt_vid_cap]
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>