[media] adv7180: Remove unnecessary v4l2_device_unregister_subdev() from probe error path

The device can't possibly be registered at this point, so no need to to call
v4l2_device_unregister_subdev().

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>