media: staging: max96712: Store format in subdev active state

Create and store the subdevice format in the subdevices active state.
This change do not have a huge effect on the driver as it do not yet
support changing the format.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>