media: staging: media: imx: imx7-media-csi: Drop unused frame_interval

The frame_interval field of the imx7_csi structure is set but never
used. Drop it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>