iio: core: move @clock_id from struct iio_dev to struct iio_dev_opaque

There is already an acessor function used to access it, making this
move straight forward.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20210426174911.397061-10-jic23@kernel.org