media: videobuf2: don't use kernel-doc "/**" markups

While it would be very cool to have those functions using
kernel-doc markups, the reality right now is that they
don't follow kernel-doc rules, as parameters aren't documented.

So, stop abusing on "/**" markups.

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