media: v4l2-async: simplify v4l2_async_subdev structure

The V4L2_ASYNC_MATCH_FWNODE match criteria requires just one
struct to be filled (struct fwnode_handle). The V4L2_ASYNC_MATCH_DEVNAME
match criteria requires just a device name.

So, it doesn't make sense to enclose those into structs,
as the criteria can go directly into the union.

That makes easier to document it, as we don't need to document
weird senseless structs.

At drivers, this makes even clearer about the match criteria.

Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Benoit Parrot <bparrot@ti.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Hyun Kwon <hyun.kwon@xilinx.com>
Acked-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>