iio:dac:ad5686: Move exports into IIO_AD5686 namespace

iio:dac:ad5686: Move exports into IIO_AD5686 namespace

Note these are used in the related ad5696-i2c drivers as well as the
more obviously connected ad5686-spi driver.

In order to avoid unnecessary pollution of the global symbol namespace
move the common/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>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220130205701.334592-8-jic23@kernel.org