media: imx: imx7_mipi_csis: Rename mipi_sd to sd

The CSIS is modelled as a single subdev, there's thus no ambiguity
regarding which subdev the code refers to. Rename mipi_sd to sd.

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>