media: staging/media/omap4iss/iss_video.c: videobuf -> vb2

Rename 'videobuf' to 'vb2' in the comment. This avoids confusing
terminology since 'videobuf' typically refers to the old vb1
framework.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>