media: adv7180: Add init_cfg pad operation

Add a init_cfg pad operation so that configurations allocated with
v4l2_subdev_alloc_pad_config() are initialized.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>