media: imx: imx7-media-csi: Rename imx7_csi_dma_start() to *_setup()

The imx7_csi_dma_start() doesn't start DMA. Rename it to
imx7_csi_dma_setup() to avoid confusion.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>