[media] Docbook: Fix comments at v4l2-async.h

Warning(.//include/media/v4l2-async.h:62): No description found for parameter 'match_type'
Warning(.//include/media/v4l2-async.h:62): Excess struct/union/enum/typedef member 'bus_type' description in 'v4l2_async_subdev'
Warning(.//include/media/v4l2-async.h:76): cannot understand function prototype: 'struct v4l2_async_notifier '

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>