Make this const as it is only passed to the const argument of the
function videobuf_queue_sg_init in the file referencing it.
Also, make the declaration in the header const.

Structure found using Coccinelle and changes done by hand.
Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>