media: v4l2-ctrls: use ref in helper instead of ctrl

The next patch needs the reference to a control instead of the
control itself, so change struct v4l2_ctrl_helper accordingly.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>