iio: cdc: ad7746: Move driver out of staging.

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>