iio:adc:ad_sigma_delta: Move exports into IIO_AD_SIGMA_DELTA namespace

iio:adc:ad_sigma_delta: Move exports into IIO_AD_SIGMA_DELTA namespace

In order to avoid unnecessary pollution of the global symbol namespace
move the core/library functions into a specific namespace and import
that into the various specific device drivers that use them.

For more information see https://lwn.net/Articles/760045/

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Renato Lui Geh <renatogeh@gmail.com>
Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220130205701.334592-2-jic23@kernel.org