All known major issues with this driver resolved so time to move
it out of staging. This also allows us to remove the now empty
staging/iio/cdc directory and build files.

Note this cleanup work was done using the roadtest framework.
https://lore.kernel.org/all/20220311162445.346685-1-vincent.whitchurch@axis.com/Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>