media: dvbdev: fix build warning due to comments

Previous commit that introduces reference counter does not add proper
comments, which will lead to warning when building htmldocs. Fix them.

Reported-by: "Stephen Rothwell" <sfr@canb.auug.org.au>
Fixes: 0fc044b2 ("media: dvbdev: adopts refcnt to avoid UAF")
Signed-off-by: Lin Ma <linma@zju.edu.cn>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>