media: Accept non-subdev sinks in v4l2_create_fwnode_links_to_pad()

The v4l2_create_fwnode_links_to_pad() helper requires the sink pad
passed to it to belong to a subdev. This requirement can be lifted
easily. Make the function usable for non-subdev sinks, which allows
using it with video_device sinks.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>