media: videodev2.h: Fix kerneldoc

Named nested unions need their prefix:
https://www.kernel.org/doc/html/latest/doc-guide/kernel-doc.html#nested-structs-unions



Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>