media: rkisp1: csi: Constify argument to rkisp1_csi_start()

The sensor argument to rkisp1_csi_start() isn't meant to be modified by
the function. Make it const.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Dafna Hirschfeld <dafna@fastmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>