[media] stk-webcam: zero the priv field of v4l2_pix_format

The priv field should be set to 0. In this case the driver abused the priv
field for internal housekeeping. Modify the code so priv is no longer used
for that purpose.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Arvydas Sidorenko <asido4@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>