media: siano: get rid of documentation warnings

The Siano driver doesn't use kernel-doc markups. While it
would be wanderful to convert to use it, it is probably
not worth the time.

So, instead of solving all problems there, just make
sure that it won't produce dozens of warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>