media: v4l2-ctrls: document nested members of structs

There are a few nested members at v4l2-ctrls.h. Now that
kernel-doc supports, document them.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>