media: imx-csi: Skip first few frames from a BT.656 source

Some BT.656 sensors (e.g. ADV718x) transmit frames with unstable BT.656
sync codes after initial power on. This confuses the imx CSI,resulting
in vertical and/or horizontal sync issues. Skip the first 20 frames
to avoid the unstable sync codes.

[fabio: fixed checkpatch warning and increased the frame skipping to 20]

Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>