[media] bw-qcam: zero priv field

Fix a v4l2-compliance problem: the priv field of v4l2_pix_format must be
cleared by drivers.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>