media: imx: imx7_mipi_csis: Don't set subdev data

The driver doesn't need to store subdev data, as the subdev is embedded
in csi_state and is thus accessed using container_of.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>