[media] coda: v4l2-compliance fix: zero pixel format priv field

If unused, the pixel format priv field has to be cleared by the driver
in try_fmt.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>