Add support for 32-bit ioctls with v4l-subdev device nodes.

Rather than keep adding new ioctls to the list in v4l2-compat-ioctl32.c, just check
if the ioctl is a non-private V4L2 ioctl and if so, call the conversion code.

We keep forgetting to add new ioctls, so this is a more robust solution.

In addition extend the subdev API with support for a compat32 function to
convert custom v4l-subdev ioctls.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>