media: i2c: thp7312: Store frame interval in subdev state

Use the newly added storage for frame interval in the subdev state to
simplify the driver.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>