media: imx: imx7-media-csi: Remove incorrect interlacing support

The driver doesn't currently support interlacing, but due to legacy
leftovers, it accepts values for the pixel format "field" field other
than V4L2_FIELD_NONE. Fix it by hardcoding V4L2_FIELD_NONE. Proper
interlacing support can be implemented later if desired.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>