media: ti: omap4iss: Fix references to pad config

V4L2 subdev operations have moved from operating on a
v4l2_subdev_pad_config to a v4l2_subdev_state a long time ago. Fix
remaining incorrect references to pad config in comments.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>