media: coda: fix unset field and fail on invalid field in buf_prepare

v4l2-compliance likes to queue a buffer with field set to V4L2_FIELD_ANY
and expects it to be returned corrected to a valid field.

Follow vicodec in handling this in the buf_prepare callback.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>